BUG: whiteboard cannot be updated

[techsmyth]

Describe the bug
The following whiteboard cannot be updated: https://alkem.io/steward-ownership-nl/challenges/fundingsoventures

To Reproduce
Open the whiteboard, make a change, and then leave.
Open it again, the change has not been saved.

Hypothesis:

• The whiteboard has a link to a document that has been deleted. That is causing an exception in the logic that is not handled.

Expected behavior
The whiteboard can be updated. Also that the image in question, that is pointing to the document that does not exist, can be deleted.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.

Areas that will be affected as part of the fix
Optional, to be added by developer if relevant