Fix Warning coming from ProcessFollowerWidget

[danielhollas]

This was missed when we changed the ProcessMonitor to use Process UUID as its traitlet. I discovered this when I was trying to get rid of warnings in unit tests in #509

@yakutovicha @unkcpz I am not very familiar with this Widget, could you please test it?

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (5309f5d) 79.31% compared to head (99bb3b4) 79.31%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #512 +/- ## ======================================= Coverage 79.31% 79.31% ======================================= Files 27 27 Lines 3834 3834 ======================================= Hits 3041 3041 Misses 793 793 ``` | [Flag](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/512/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | Coverage Δ | | |---|---|---| | [python-3.10](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/512/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | `79.31% <ø> (ø)` | | | [python-3.8](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/512/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | `79.35% <ø> (ø)` | | | [python-3.9](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/512/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | `79.35% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/512?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab) | Coverage Δ | | |---|---|---| | [aiidalab\_widgets\_base/process.py](https://app.codecov.io/gh/aiidalab/aiidalab-widgets-base/pull/512?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidalab#diff-YWlpZGFsYWJfd2lkZ2V0c19iYXNlL3Byb2Nlc3MucHk=) | `78.58% <ø> (ø)` | |

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[yakutovicha]

@unkcpz, since you are reviewing it, I will un-assign myself. I trust you guys :)