harden GeneratedGraphicItems methods

[mjkkirschner]

Purpose

I was not able to reproduce this crash, but my guess is that somehow engineController has been disposed/set to null temporarily, which does happen from time to time when reseting the vm, opening a new graph, or at shutdown.

Check for nulls, and bail.

System.NullReferenceException in
 Dynamo.Graph.Nodes.NodeModelExtensions.GeneratedGraphicItems(NodeModel node,
EngineController engineController) in System.Linq.Enumerable.<SelectManyIterator>d__17`2.MoveNext() in
Dynamo.Applications.ViewModel.RevitWatch3DViewModel.Draw(NodeModel node) in
System.EventHandler`1.Invoke(Object sender, TEventArgs e) in
Dynamo.Models.DynamoModel.OnEvaluationCompleted(Object sender, EvaluationCompletedEventArgs e) in
System.EventHandler`1.Invoke(Object sender, TEventArgs e) in
Dynamo.Graph.Workspaces.HomeWorkspaceModel.OnEvaluationCompleted(EvaluationCompletedEventArgs e) in 

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

[sm6srw]

LGTM!

[pinzart90]

EngineController looks like the main suspect
https://git.autodesk.com/Dynamo/DynamoRevit/blob/14cf7520884a2e285c81241847097fccf1cd6999/src/DynamoRevit/ViewModel/RevitWatch3DViewModel.cs#L121-L133

And I do see a lot of code in Dynamo checks if the controller is null (maybe some graphics are generated while the DynamoModel is being disposed ?...)