Structure_Engine: NullReferenceException on transform of a bar with null nodes fixed

[pawelbaran]

Issues addressed by this PR

Closes #3029

Test files

On SharePoint.

Changelog

Additional comments

[IsakNaslundBh]

Happy to approve.

Null handling and messages makes sense.

Happy for this to be merged to be included in 6.1 beta as a really low-risk PR only adding null-checking. Not required for 6.1 release, though, so can wait until 6.2 milestone if that is preferred by DevOps. @FraserGreenroyd

[FraserGreenroyd]

@BHoMBot check compliance
@BHoMBot check required

[bhombot-ci]

@FraserGreenroyd to confirm, the following actions are now queued:

• check code-compliance
• check documentation-compliance
• check project-compliance
• check branch-compliance
• check dataset-compliance
• check copyright-compliance
• check code-compliance
• check documentation-compliance
• check project-compliance
• check core
• check null-handling
• check serialisation
• check versioning
• check installer

There are 8 requests in the queue ahead of you.

[bhombot-ci]

The check code-compliance has already been run previously and recorded as a successful check. This check has not been run again at this time.

[bhombot-ci]

The check documentation-compliance has already been run previously and recorded as a successful check. This check has not been run again at this time.

[FraserGreenroyd]

@BHoMBot check unit-tests

[bhombot-ci]

@FraserGreenroyd to confirm, the following actions are now queued:

• check unit-tests

There are 2 requests in the queue ahead of you.

[FraserGreenroyd]

@BHoMBot check versioning

[bhombot-ci]

@FraserGreenroyd to confirm, the following actions are now queued:

• check versioning

There are 2 requests in the queue ahead of you.

[FraserGreenroyd]

@BHoMBot check ready-to-merge

[bhombot-ci]

@FraserGreenroyd to confirm, the following actions are now queued:

• check ready-to-merge

There are 11 requests in the queue ahead of you.