Fix 'vakata-contextmenu should be visible' (rebased onto develop)


This is the same as gh-3137 but rebased onto develop.

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Trying to fix errors from https://ci.openmicroscopy.org/view/Failing/job/OMERO-5.0-merge-robotframework/200/console
```
08:09:25 Test Container Creation Enabled :: Select User, Project, Dataset, ... | FAIL |
08:09:25 The element 'vakata-contextmenu' should be visible, but it is not.
```