Added assertions which ensure that we are in the UI thread.

Author: joachimmarder

Source/VirtualTrees.pas

@@ -8383,6 +8383,7 @@ procedure TVirtualTreeColumns.UpdatePositions(Force: Boolean = False);
 function TVirtualTreeColumns.Add: TVirtualTreeColumn;
 
 begin
+  Assert(GetCurrentThreadId = MainThreadId, 'UI controls may only be chnaged in UI thread.');
   Result := TVirtualTreeColumn(inherited Add);
 end;
 
@@ -29811,6 +29812,7 @@ function TBaseVirtualTree.InvalidateNode(Node: PVirtualNode): TRect;
 
 begin
   Assert(Assigned(Node), 'Node must not be nil.');
+  Assert(GetCurrentThreadId = MainThreadId, 'UI controls may only be chnaged in UI thread.');
   // Reset height measured flag too to cause a re-issue of the OnMeasureItem event.
   Exclude(Node.States, vsHeightMeasured);
   if (FUpdateCount = 0) and HandleAllocated then