Make sure the citation dialog isn't just appear behind your other windows. That's the most likely cause of this. The citation dialog window is part of the Zotero app.
If you're sure it's not that, try disabling all your browser extensions (particularly Google Docs Offline) and reload Google Docs, and then try again.
Upgrade Storage
If you're sure it's not that, try disabling all your browser extensions (particularly Google Docs Offline) and reload Google Docs, and then try again.