How are you triggering this bug? this.model should be a ve.dm.Node which implements the method.

I have a VE plugin for SemanticMediaWiki extension that calls getFragment()
See https://en.wikipedia.org/wiki/User:Nischayn22/veSMW.js

Firstly instead of overriding core classes, you should be creating subclasses and registering those over the originals. I'm not sure what you need the fragment for at this point, but there are other places to get it depending on when you need it.

I understand that's more of a comment on the code rather than this bug. I code is very messy, I need to fix it but first I need to make it working.

Change 223817 had a related patch set uploaded (by Esanders):
Add checks to ve.ui.ContextItem#getFragment

https://gerrit.wikimedia.org/r/223817

Change 223817 merged by jenkins-bot:
Add checks to ve.ui.ContextItem#getFragment

https://gerrit.wikimedia.org/r/223817