Cherry-pick of #16406 to RC3.6.0_master

Purpose

Under some order of operations, nodes with a null model are generating a null reference exception when dragged into groups. This PR prevents adding null nodes and transient nodes into groups.

Declarations

Check these if you believe they are true

• 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
• 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
• This PR introduces new feature code involve network connecting and is tested with no-network mode.

Release Notes

Prevent adding null nodes and transient nodes into groups.

Reviewers

@DynamoDS/synapse

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