Apple Mail blank signature dialogue box, no options to change apple mail signatures

My Apple Mail app (even after updating) is still causing me a bug, whereby when I want to add or change a signature, the dialogue screen that pops up is just completely blank, with no txt, or buttons, or anything.

I can only close the dialogue by using the MacOS red close windo button in the top-left hand side. The dialogue is compeltely empty.

No matter how the signatures are accessed, either through Mail preferences, MacOs preferences or by clicking on 'Edit Signatures' in a new email message, the same blank window appears.

Please help!