Fix Dynamo crash after long idle

[QilongTang]

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Purpose

After long idle, Dynamo will lose geometry background preview and clicking on canvas will crash Dynamo. The reason is that certain cleanup code is related to workspace view unloaded event which could be triggered in cases where Dynamo idle for long.

The code in question are introduced as part of #2888.

The fix in this PR is to move clean up code for workspace in OnWorkspaceViewDataContextChanged because this should be enough to prevent memory leak.

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

Reviewers

@DynamoDS/dynamo

FYIs

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[QilongTang]

The OnWorkspaceViewDataContextChanged will be triggered in both cases so I do not see them as must have.

[zeusongit]

LGTM