FIX: check we have a back button in tk toolbar before we touch it

[tacaswell]

PR Summary

We add one by default, but sub-classes may remove it.

closes #18232

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant

[lumberbot-app]

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

$ git checkout v3.3.x
$ git pull

2. Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 03753154ef2d555bc2badfc75defa62b653fe30c

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #18235: FIX: check we have a back button in tk toolbar before we touch it'

4. Push to a named branch :

git push YOURFORK v3.3.x:auto-backport-of-pr-18235-on-v3.3.x

5. Create a PR against branch v3.3.x, I would have named this PR:

"Backport PR #18235 on branch v3.3.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.