Fix #6725: Allow theme without second topbar menu

[marc-farre]

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the develop branch, not the master branch if no hotfix
• When resolving a specific issue, it's referenced in the PR's description (e.g. Fix #xxx[,#xxx], where "xxx" is the Github issue number)
• All tests are passing
• New/updated tests are included
• Changelog was modified

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

Issue #6725