Fix Vega Scene Graph Errors after Confirming Output Cohorts

[keckelt]

Closes https://github.com/Caleydo/cohort/issues/720

Developer Checklist (Definition of Done)

Issue

• All acceptance criteria from the issue are met
• Tested in latest Chrome/Firefox

UI/UX/Vis

• Requires UI/UX/Vis review
  • Reviewer(s) are notified (tag assignees)
  • Review has occurred (link to notes)
  • Feedback is included in this PR
  • Reviewer(s) approve of concept and design

Code

• Branch is up-to-date with the branch to be merged with, i.e., develop
• Code is cleaned up and formatted
• Unit tests are written (frontend/backend if applicable)
• Integration tests are written (if applicable)

PR

• Descriptive title for this pull request is provided (will be used for release notes later)
• Reviewer and assignees are defined
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• Add release label (e.g., release: minor) to this PR following semver
• The PR is connected to the corresponding issue (via Closes #...)
• Summary of changes is written

Summary of changes

• Searchbar: Update Tasks when searchbar is cleared
• AttributeColumn/MiniVisualization: Remove Vega visualizations properly if column is removed (didn't throw an error yet, but still)
• Filter Task: The vega view will be destroyed as soon as show is called, as it will be replaced/removed anyway.
  the second call of this.vis.destroy() in Filter.ts is still necessary, in case the user changes the visualization (e.g., Kaplan-Meier to Density)

Screenshots

Additional notes for the reviewer(s)

Thanks for creating this pull request 🤗