Repro steps:
- open the attached project in Unity 2018.3.0a7

When updating a project that uses TMP Pro from 2018.2 to 2018.3 0a7, I have been getting my console spammed by errors from the TMP package:
NullReferenceException: Object reference not set to an instance of an object
TMPro.PackageResourceImporterWindow.GetGUIStyles () (at Library/PackageCache/com.unity.textmeshpro@1.2.4/Scripts/Runtime/PackageResourceImporterWindow.cs:139)
TMPro.PackageResourceImporterWindow.OnEnable () (at Library/PackageCache/com.unity.textmeshpro@1.2.4/Scripts/Runtime/PackageResourceImporterWindow.cs:41)
UnityEditor.EditorWindow:GetWindow()
TMPro.PackageResourceImporterWindow:ShowPackageImporterWindow() (at Library/PackageCache/com.unity.textmeshpro@1.2.4/Scripts/Runtime/PackageResourceImporterWindow.cs:14)
TMPro.TMP_Settings:get_instance() (at Library/PackageCache/com.unity.textmeshpro@1.2.4/Scripts/Runtime/TMP_Settings.cs:297)
TMPro.TextMeshProUGUI:Awake() (at Library/PackageCache/com.unity.textmeshpro@1.2.4/Scripts/Runtime/TMPro_UGUI_Private.cs:77)

Also, the TMP Importer window appears and, if you chose to close it, it will appear all the time when entering play mode, and this message spams the console
ArgumentException: Getting control 0's position in a group with only 0 controls when doing Repaint
Aborting
UnityEngine.GUILayoutGroup.GetNext () (at C:/buildslave/unity/build/Modules/IMGUI/LayoutGroup.cs:117)
UnityEngine.GUILayoutUtility.DoGetRect (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Modules/IMGUI/GUILayoutUtility.cs:425)
UnityEngine.GUILayoutUtility.GetRect (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Modules/IMGUI/GUILayoutUtility.cs:385)
UnityEngine.GUILayout.DoLabel (UnityEngine.GUIContent content, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Modules/IMGUI/GUILayout.cs:16)
UnityEngine.GUILayout.Label (System.String text, UnityEngine.GUIStyle style, UnityEngine.GUILayoutOption[] options) (at C:/buildslave/unity/build/Modules/IMGUI/GUILayout.cs:12)
TMPro.PackageResourceImporterWindow.OnGUI () (at Library/PackageCache/com.unity.textmeshpro@1.2.1/Scripts/Runtime/PackageResourceImporterWindow.cs:65)
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:222)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MonoMethod.cs:232)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Reflection/MethodBase.cs:115)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:335)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:329)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition, Rect viewRect) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:295)

all of these messages go away when the Text Mesh Pro package gets removed from the project.