Fix/53-webview-crash-context-menu

[pogi7]

Checklist before submitting a merge request

• [x] I have reviewed CONTRIBUTING.md document
• [x] All commits have been signed-off for the Developer Certificate of Origin. See here.
• [x] I have added my contribution to CHANGELOG.md
• [x] I have documented my code in the OML Vision Docs

Description of contribution

This fixes the bug where a webview (Table, Tree, or Diagram) crashes when a context menu is not defined. Bug occurs when a user right clicks in the webview. closes #53

Testing performed

• [x] I have added unit tests to cover additional functionality (Frontend tests still need to be added)

How to Test Expected functionality changes

[Describe expected behaviour changes in steps]

1. Go to https://github.com/UTNAK/kepler16b-using-imce-vocabulary
2. Switch to main branch
3. Run git pull
4. From the command line run ./gradlew load or from the Setup Tasks webview
5. Wait for that command to finish
6. Open the decompositions-autogen diagram webview
7. Right click on any node
8. Verify that the diagram webview doesn't crash
9. Open the Objectives table webview
10. Right click on any row
11. Verify that the table webview doesn't crash
12. Open the Requirements tree webview
13. Right click on any row
14. Verify that the tree webview doesn't crash

Additional context

[If needed, you may add additional context]