Fixed crash on right-click without modal, introduced by previous commit

a3667f4,  (ocornut#4729)

imgui.cpp

@@ -3712,7 +3712,7 @@ void ImGui::UpdateMouseMovingWindowEndFrame()
         // Find the top-most window between HoveredWindow and the top-most Modal Window.
         // This is where we can trim the popup stack.
         ImGuiWindow* modal = GetTopMostPopupModal();
-        bool hovered_window_above_modal = g.HoveredWindow && IsWindowAbove(g.HoveredWindow, modal);
+        bool hovered_window_above_modal = g.HoveredWindow && (modal == NULL || IsWindowAbove(g.HoveredWindow, modal));
         ClosePopupsOverWindow(hovered_window_above_modal ? g.HoveredWindow : modal, true);
     }
 }