Kanban board Javascript error: Uncaught TypeError: Cannot read property "id" of null in com.pyxis.greenhoper.jira:..

We got sometimes errors, e.g. if we want assign a user at the kanban board.

Message looks like:
Exception: Uncaught TypeError: Cannot read property "id" of null in com.pyxis.greenhoper.jira:...
Line 914
Column: 797