Loading the exercise page when not signed in causes JS errors

[sentry-io]

If you open an exercise page when not signed in, the submit button is not shown (this is by design). However, some code related to setting up this button now fails causing JS errors. I added this github issue to one of the sentry errors, but multiple errors will probably be solved by this.

To reproduce: open an exercise page when not signed in and click the submissions tab.

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Sentry Issue: DODONA-FRONTEND-14

TypeError: null is not an object (evaluating 'document.getElementById("activity-handin-link").addEventListener')
  at swapActionButtons (./app/assets/javascripts/exercise.ts:201:33)
  at init (./app/assets/javascripts/exercise.ts:152:13)
  at initEditor (./app/assets/javascripts/exercise.ts:193:5)
  at initExerciseShow (./app/assets/javascripts/exercise.ts:466:5)
  at afterResize (./app/assets/javascripts/exercise.ts:469:1)
...
(2 additional frame(s) were not displayed)

[jorg-vr]

The other enableSubmitButton issues don't seem to be related to this case...

I will research them further