incorrect behaviour during export as image

NOTE: Errors mentioned in the below list: 5., 10. and 12. seems to occur only on first map browsing. I couldn't reproduce them again on below map (test-submaps-map__NEW) but when I uploaded that map again - these errors occurred.

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checked on: https://pg-sandbox.uni.lu/minerva/index.xhtml?id=test-submaps-map__NEW
source file: test-submaps-map.zip

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I described what I did and what was result of action. The issue is related to export as map/image and overlays.

1. Open/Display a map (first opening ever).

2. Export current view as map --> occurs in: success but white&black colours of elements.

3. Export as map CellD --> occurs in: map exported, but black and white elements.

4. Zoom in map.

5. Export as image occurs with error: {"reason":"deleted object would be re-saved by cascade (remove deleted object from associations): [lcsb.mapviewer.model.map.MiriamData#110457]","error":"Internal server error."}

6. Choosing 'Select mode' and export as image - occurs with the same above error.

7. adding data-overlay --> display overlay --> map is coloured.

8. Export as image --> occurs in {"error":"Internal server error."}

9. Turn off overlay, back to Pathway and Compartments view.

10. Export as map ---> occurs in: {"reason":"deleted object would be re-saved by cascade (remove deleted object from associations): [lcsb.mapviewer.model.map.MiriamData#110461]","error":"Internal server error."}

11. remove overlay

12. export as map ---> occurs in: {"reason":"deleted object would be re-saved by cascade (remove deleted object from associations): [lcsb.mapviewer.model.map.MiriamData#110461]","error":"Internal server error."}

13. After few minutes: magicaly the export as map/image works - and the exported image/map is white black with data-overlays colours

14. Also, sometimes after unsuccessful export http error 500 happens, I submitted it by minerva-reporter #1502 (closed) #1500 (closed).