We've just investigated your file, but we couldn't find any problem in your file.
One possible scenario why the diagram is disappeared is:
1. Clicked on empty area of the diagram. It resulted selecting the diagram.
2. Pressed Ctrl+Delete (Delete from Model menu). It resulted the diagram removal.
3. Saved the file.
We just realized that the Step 2 do not close the working diagram, so user may not recognize the deletion of diagram.
So, we will fix this problem and consider not to select the diagram when clicking on the empty area of the diagram.
We want to let you know how to create a diagram quickly instead of drag-and-drop from every elements in Explorer on a diagram.
1. Open your file (Untitled.mdj)
2. Select "Model" element in Explorer.
3. Click the menu (Tools > Diagram Generator > Overview (Expanded)). A diagram "Overview" will be created under "Model" element.
4. Double-click the "Overview" diagram in Explorer. You can see the created diagram.
5. Re-layout your view elements in the created diagram.
Anyway, we're sorry for inconvenience. we will enhance to prevent facing this situation again.