Temporary connector leak bugfix

[bendikberg]

Purpose

If the process of creating a new connector is cancelled by the user, the temporary ConnectorViewModel is not disposed. It then hangs around listening for canvas events.

Adding the .Dispose() call when the connector is removed from WorkspaceElements fixes this, but the temporary connector does not have a model set, so the .Dispose function must also be changed to accomodate this.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• This PR modifies some build requirements and the readme is updated
• This PR contains no files larger than 50 MB

Release Notes

N/A

Reviewers

@mjkkirschner

FYIs

@dimven