To reproduce:
1. Open attached project
2. Press on E-Star in the hierarchy
3. Press on Start Color and change the gradient

Actual result: Error of NullreferenceException appears.
Expected result: There should be no error.

Error:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.EyeDropper.End () (at C:/buildslave/unity/build/Editor/Mono/GUI/ColorPicker.cs:1282)
UnityEditor.EditorGUI.DoColorField (UnityEngine.Rect position, System.Int32 id, UnityEngine.Color value, System.Boolean showEyedropper, System.Boolean showAlpha, System.Boolean hdr) (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:4433)
UnityEditor.EditorGUI.ColorField (UnityEngine.Rect position, UnityEngine.GUIContent label, UnityEngine.Color value, System.Boolean showEyedropper, System.Boolean showAlpha, System.Boolean hdr) (at C:/buildslave/unity/build/Editor/Mono/EditorGUI.cs:4304)
UnityEditor.GradientEditor.OnGUI (UnityEngine.Rect position) (at C:/buildslave/unity/build/Editor/Mono/GUI/GradientEditor.cs:203)
UnityEditor.GradientPicker.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/GradientPicker.cs:202)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <1f0c1ef1ad524c38bbc5536809c46b48>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <1f0c1ef1ad524c38bbc5536809c46b48>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <1f0c1ef1ad524c38bbc5536809c46b48>:0)
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.OldOnGUI () (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:141)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:295)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:481)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:464)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent (UnityEngine.Event e, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:492)
UnityEngine.UIElements.IMGUIContainer.HandleIMGUIEvent () (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:455)
UnityEngine.UIElements.UIR.ImmediateRenderer.Draw (UnityEngine.UIElements.UIR.DrawChainState dcs) (at C:/buildslave/unity/build/Modules/UIElements/Renderer/UIRenderer/UIRenderers.cs:447)
UnityEngine.UIElements.UIR.UIRenderDevice.ContinueChain (UnityEngine.UIElements.UIR.RendererBase head, UnityEngine.UIElements.UIR.DrawChainState dcs, System.Boolean outerChainsWithMeshRenderer) (at C:/buildslave/unity/build/Modules/UIElements/Renderer/UIRenderer/UIRenderDevice.cs:865)
UnityEngine.UIElements.UIR.UIRenderDevice.DrawChain (UnityEngine.UIElements.UIR.RendererBase head, UnityEngine.Rect viewport, UnityEngine.Matrix4x4 projection, UnityEngine.Texture atlas) (at C:/buildslave/unity/build/Modules/UIElements/Renderer/UIRenderer/UIRenderDevice.cs:848)
UnityEngine.UIElements.UIRRepaintUpdater.DrawChain (UnityEngine.Rect topRect, UnityEngine.Matrix4x4 projection) (at C:/buildslave/unity/build/Modules/UIElements/Renderer/UIRRepaintUpdater.cs:218)
UnityEngine.UIElements.UIRRepaintUpdater.Update () (at C:/buildslave/unity/build/Modules/UIElements/Renderer/UIRRepaintUpdater.cs:190)
UnityEngine.UIElements.VisualTreeUpdater.UpdateVisualTree () (at C:/buildslave/unity/build/Modules/UIElements/VisualTreeUpdater.cs:70)
UnityEngine.UIElements.Panel.Repaint (UnityEngine.Event e) (at C:/buildslave/unity/build/Modules/UIElements/Panel.cs:589)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:240)
UnityEngine.UIElements.UIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/UIElements/UIElementsUtility.cs:78)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at C:/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:179)

Reproduced with: 2018.3.14f1; 2018.4.3f1; 2019.1.8f1; 2019.2.0b7; 2019.3.0a8
Not reproduce on: 2017.4.29f1