Fix errors in handling pause, and MD workspaces with no UB

[AndreiSavici]

Short description of the changes:

Pause was not handled correctly. In the test it would have killed all events, and it would crash on ConvertToMD.
The sample dialog would crash if no UB is already present in the workspace

Long description of the changes:

Check list for the pull request

• I have read the [CONTRIBUTING]
• I have read the [CODE_OF_CONDUCT]
• I have added tests for my changes
• I have updated the documentation accordingly

Check list for the reviewer

• I have read the [CONTRIBUTING]
• I have verified the proposed changes
• best software practices
  • all internal functions have an underbar, as is python standard
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

Manual test for the reviewer

References

[mpatrou]

It looks good to me!!