add null check to notifications center location change handler.

[mjkkirschner]

Purpose

Looking at common CER crashes for 2.16 I notice multiple crashes for a null reference exception when the dynamo window is moved.
It appears that in rare circumstances you can move the Dynamo window before the NotificationsCenter constructor is finished executing, this will kick off the location changed handler, which will try to access the UI popup - but it has not been created yet.

Just add a null check.

Object reference not set to an instance of an object.

   at Dynamo.Notifications.NotificationCenterController.DynamoView_LocationChanged(Object sender, EventArgs e)
   at System.EventHandler.Invoke(Object sender, EventArgs e)
   at System.Windows.Window.OnLocationChanged(EventArgs e)
   at System.Windows.Window.WmMoveChangedHelper()
   at System.Windows.Window.WmMoveChanged()
   at System.Windows.Window.WindowFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at System.Windows.Interop.HwndSource.PublicHooksFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
   at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Int32 numArgs

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated

Release Notes

Fix crash when moving Dynamo Window.

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[mjkkirschner]

The failure is just DynamoCoreWpfTests.CoreUserInterfaceTests.WorkspaceContextMenu_TestIfInCanvasSearchHidesOnOpeningContext again

[RobertGlobant20]

LGTM