Steps to repro:
# create a new project in unity
# open the ui builder and add a visual element to the document
# drag and drop an image file in unity's project window
# The UI Builder asks to save the file ( first issue )
# save the file ( see attached gif )

Result: Calls to "AssetDatabase.Refresh" are restricted during asset importing. 
UnityEngine.StackTraceUtility:ExtractStackTrace () (at C:/Mercurial/Beta/unity/Runtime/Export/Scripting/StackTrace.cs:37)
UnityEditor.AssetDatabase:Refresh () (at C:/Mercurial/Beta/unity/Modules/AssetDatabase/Editor/ScriptBindings/AssetDatabase.bindings.cs:306)
Unity.UI.Builder.BuilderDocumentOpenUXML:SaveNewDocument (UnityEngine.UIElements.VisualElement,bool,bool&,string) (at C:/Mercurial/Beta/unity/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Document/BuilderDocumentOpenUXML.cs:452)
Unity.UI.Builder.BuilderDocument:SaveNewDocument (UnityEngine.UIElements.VisualElement,bool,bool&,string) (at C:/Mercurial/Beta/unity/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Document/BuilderDocument.cs:272)
Unity.UI.Builder.BuilderToolbar:SaveDocument (bool) (at C:/Mercurial/Beta/unity/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Toolbar/BuilderToolbar.cs:376)
Unity.UI.Builder.Builder:SaveChanges () (at C:/Mercurial/Beta/unity/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Builder.cs:218)
Unity.UI.Builder.BuilderAssetModificationProcessor:OnWillSaveAssets (string[]) (at C:/Mercurial/Beta/unity/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Utilities/BuilderAssetModificationProcessor.cs:88)
System.Reflection.RuntimeMethodInfo:Invoke (object,System.Reflection.BindingFlags,System.Reflection.Binder,object[],System.Globalization.CultureInfo)
System.Reflection.MethodBase:Invoke (object,object[])
UnityEditor.AssetModificationProcessorInternal:OnWillSaveAssets (string[],string[]&,string[]&,bool) (at C:/Mercurial/Beta/unity/Editor/Mono/AssetModificationProcessor.cs:165)
UnityEditor.DragAndDrop:DefaultProjectBrowserDropHandlerImpl (int,string,bool) (at C:/Mercurial/Beta/unity/Editor/Mono/DragAndDrop.bindings.cs:147)
System.Reflection.RuntimeMethodInfo:Invoke (object,System.Reflection.BindingFlags,System.Reflection.Binder,object[],System.Globalization.CultureInfo)
System.Reflection.MethodBase:Invoke (object,object[])
System.Delegate:DynamicInvokeImpl (object[])
System.MulticastDelegate:DynamicInvokeImpl (object[])
System.Delegate:DynamicInvoke (object[])
UnityEditor.DragAndDrop:Drop (int,object[]) (at C:/Mercurial/Beta/unity/Editor/Mono/DragAndDrop.bindings.cs:132)
UnityEditor.DragAndDrop:DropOnProjectBrowserWindow (int,string,bool) (at C:/Mercurial/Beta/unity/Editor/Mono/DragAndDrop.bindings.cs:101)
UnityEditor.ObjectListArea/LocalGroup:DoDrag (int,bool) (at C:/Mercurial/Beta/unity/Editor/Mono/ObjectListLocalGroup.cs:1419)
UnityEditor.ObjectListArea/LocalGroup:HandleUnusedDragEvents (single) (at C:/Mercurial/Beta/unity/Editor/Mono/ObjectListLocalGroup.cs:313)
UnityEditor.ObjectListArea/Group:Draw (single,UnityEngine.Vector2,int&) (at C:/Mercurial/Beta/unity/Editor/Mono/ObjectListGroup.cs:159)
UnityEditor.ObjectListArea:HandleListArea () (at C:/Mercurial/Beta/unity/Editor/Mono/ObjectListArea.cs:1090)
UnityEditor.ObjectListArea:OnGUI (UnityEngine.Rect,int) (at C:/Mercurial/Beta/unity/Editor/Mono/ObjectListArea.cs:396)
UnityEditor.ProjectBrowser:OnGUI () (at C:/Mercurial/Beta/unity/Editor/Mono/ProjectBrowser.cs:2061)
UnityEditor.HostView:InvokeOnGUI (UnityEngine.Rect) (at C:/Mercurial/Beta/unity/Editor/Mono/HostView.cs:474)
UnityEditor.DockArea:DrawView (UnityEngine.Rect) (at C:/Mercurial/Beta/unity/Editor/Mono/GUI/DockArea.cs:387)
UnityEditor.DockArea:OldOnGUI () (at C:/Mercurial/Beta/unity/Editor/Mono/GUI/DockArea.cs:378)
UnityEngine.UIElements.IMGUIContainer:DoOnGUI (UnityEngine.Event,UnityEngine.Matrix4x4,UnityEngine.Rect,bool,UnityEngine.Rect,System.Action,bool) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/IMGUIContainer.cs:353)
UnityEngine.UIElements.IMGUIContainer:HandleIMGUIEvent (UnityEngine.Event,UnityEngine.Matrix4x4,UnityEngine.Rect,System.Action,bool) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/IMGUIContainer.cs:673)
UnityEngine.UIElements.IMGUIContainer:HandleIMGUIEvent (UnityEngine.Event,System.Action,bool) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/IMGUIContainer.cs:645)
UnityEngine.UIElements.IMGUIContainer:HandleIMGUIEvent (UnityEngine.Event,bool) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/IMGUIContainer.cs:638)
UnityEngine.UIElements.IMGUIContainer:SendEventToIMGUIRaw (UnityEngine.UIElements.EventBase,bool,bool) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/IMGUIContainer.cs:598)
UnityEngine.UIElements.IMGUIContainer:SendEventToIMGUI (UnityEngine.UIElements.EventBase,bool,bool) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/IMGUIContainer.cs:587)
UnityEngine.UIElements.IMGUIContainer:ProcessEvent (UnityEngine.UIElements.EventBase) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/IMGUIContainer.cs:511)
UnityEngine.UIElements.CallbackEventHandler:HandleEvent (UnityEngine.UIElements.EventBase) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/Events/EventHandler.cs:267)
UnityEngine.UIElements.CallbackEventHandler:HandleEventAtCurrentTargetAndPhase (UnityEngine.UIElements.EventBase) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/Events/EventHandler.cs:189)
UnityEngine.UIElements.CallbackEventHandler:UnityEngine.UIElements.IEventHandler.HandleEvent (UnityEngine.UIElements.EventBase) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/Events/EventHandler.cs:206)
UnityEngine.UIElements.EventDispatchUtilities:HandleEventAcrossPropagationPath (UnityEngine.UIElements.EventBase) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/Events/IEventDispatchingStrategy.cs:143)
UnityEngine.UIElements.EventDispatchUtilities:PropagateEvent (UnityEngine.UIElements.EventBase) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/Events/IEventDispatchingStrategy.cs:90)
UnityEngine.UIElements.MouseEventDispatchingStrategy:SendEventToRegularTarget (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/Events/MouseEventDispatchingStrategy.cs:36)
UnityEngine.UIElements.MouseEventDispatchingStrategy:SendEventToTarget (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.BaseVisualElementPanel) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/Events/MouseEventDispatchingStrategy.cs:26)
UnityEngine.UIElements.MouseEventDispatchingStrategy:DispatchEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.IPanel) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/Events/MouseEventDispatchingStrategy.cs:19)
UnityEngine.UIElements.EventDispatcher:ApplyDispatchingStrategies (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.IPanel,bool) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/EventDispatcher.cs:422)
UnityEngine.UIElements.EventDispatcher:ProcessEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.IPanel) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/EventDispatcher.cs:363)
UnityEngine.UIElements.EventDispatcher:Dispatch (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.IPanel,UnityEngine.UIElements.DispatchMode) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/EventDispatcher.cs:229)
UnityEngine.UIElements.BaseVisualElementPanel:SendEvent (UnityEngine.UIElements.EventBase,UnityEngine.UIElements.DispatchMode) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/Panel.cs:467)
UnityEngine.UIElements.UIElementsUtility:DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/UIElementsUtility.cs:476)
UnityEngine.UIElements.UIElementsUtility:UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (int,intptr,bool&) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/UIElementsUtility.cs:205)
UnityEngine.UIElements.UIEventRegistration:ProcessEvent (int,intptr) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/UIElementsUtility.cs:74)
UnityEngine.UIElements.UIEventRegistration/<>c:<.cctor>b__1_2 (int,intptr) (at C:/Mercurial/Beta/unity/ModuleOverrides/com.unity.ui/Core/UIElementsUtility.cs:28)
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&) (at C:/Mercurial/Beta/unity/Modules/IMGUI/GUIUtility.cs:189)
[C:/Mercurial/Beta/unity/Runtime/Export/Scripting/StackTrace.cs line 37]