During a signing process, currently the "Save" and "Save as ..." actions can be selected.

On "Save":

• Nothing happens (also no warning/error)

On "Save as...":

• Default is the original filename
• If the original filename is overwritten
  • signature placeholder fields are created and a banner is displayed:
    
  • selecting "Finish" and trying to save afterwards (different filename) results in an Okular crash

I'd suggest to just deactivate "Save" and "Save as" during the signing process.
Alternative: Start "Finish" on "Save as".