Fix closing invention list window while holding invention

Calling `Close()` on a window deletes its object, rendering any future
uses of its members invalid.

This can be triggered by opening inventions list window, holding an
invention, closing the inventions list window (e.g. with keyboard) and
then releasing the held invention

src/openrct2-ui/windows/EditorInventionsList.cpp

@@ -619,6 +619,7 @@ public:
         if (inventionListWindow == nullptr)
         {
             Close();
+            return;
         }
         std::optional<InventionListItem> res;
         // Skip always researched items, so that the dragged item gets placed underneath them