Something like a gallery or document list block is using a file set that has since been deleted and the block's code is not bomb-proofed against such a situation.

There may a bug report on github about it for one of the core blocks.

If you go to the dashboard debug settings and enable the full debug to screen, you will get a backtrace with line numbers you can use to identify the offending block code.

The actual error could be a little earlier, where getFileSetByID is called on the missing file set ID.

Then its a matter of testing if the file set is an object and short circuiting whatever follows so it can fail without breaking the page.

Many thanks

Diamond !