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When retrieving a machine property in the Workflow Designer of vCAC that does not exist and later on access the internal workflow variable holding that machine property value you will receive a “System.NullReferenceException”.

[UTC:21/07/13 13:11:30.255 Local:21/07 15:11] [Error]: <?xml version="1.0" encoding="utf-16"?>
<boolean>false</boolean>
Workflow "WFStubMachineProvisioned" failed with the following exception:
System.NullReferenceException: Object reference not set to an instance of an object.
   at lambda_method(Closure , ActivityContext )
   at Microsoft.VisualBasic.Activities.VisualBasicValue`1.Execute(CodeActivityContext context)
   at System.Activities.CodeActivity`1.InternalExecute(ActivityInstance instance, ActivityExecutor executor, BookmarkManager bookmarkManager)
   at System.Activities.Runtime.ActivityExecutor.ExecuteActivityWorkItem.ExecuteBody(ActivityExecutor executor, BookmarkManager bookmarkManager, Location resultLocation)

When using a “TryCatch” activity to react upon this condition an exception is only triggered when you set “Is Required” to “True” (regardless of what you defined in your catch block).

As a workaround for you can also pre-initialise the variables in your workflow and then you might not have to use the “TryCatch” activity at all.