jvcllaz: Fix TJvDBTreeView and TJvDBLookupTreeView crashing the IDE when they are deleted from the form.

git-svn-id: https://svn.code.sf.net/p/lazarus-ccr/svn@6871 8e941d3f-bd1b-0410-a28a-d453659cc2b4

components/jvcllaz/run/JvDB/JvDBTreeView.pas

@@ -1581,7 +1581,9 @@ var
   strLength: Integer;
   HasChildren: Byte;
 begin
-  if Items.Count > 0 then
+  // wp: not clear if this still works correctly: had to add "Assigned(Items)"
+  // to prevent crash when destroying
+  if Assigned(Items) and (Items.Count > 0) then
   begin
     // save master values into stream
     FMastersStream := TMemoryStream.Create;