Exception from multithreaded Xcode build

[markfinal]

This manifests in a number of ways, but generally always like this

ERROR: (Bam.Core.Exception) Error during threaded build
ERROR: (System.AggregateException) One or more errors occurred.
ERROR: (System.InvalidOperationException) Collection was modified; enumeration operation may not execute.
ERROR:
at System.ThrowHelper.ThrowInvalidOperationException (ExceptionResource resource) [0x00000] in /private/tmp/source-mono-mac-4.2.0-branch/bockbuild-mono-4.2.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.2.1/external/referencesource/mscorlib/system/throwhelper.cs:94
at System.Collections.Generic.List1+Enumerator[T].MoveNextRare () <0x17b9ad0 + 0x00031> in <filename unknown>:0 at System.Collections.Generic.List1+Enumerator[T].MoveNext () <0x17b9a30 + 0x00080> in :0
at System.Linq.Enumerable+WhereListIterator1[TSource].MoveNext () [0x00062] in /private/tmp/source-mono-mac-4.2.0-branch/bockbuild-mono-4.2.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.2.1/external/referencesource/System.Core/System/Linq/Enumerable.cs:230 at System.Linq.Enumerable.FirstOrDefault[TSource] (IEnumerable1 source) [0x0003e] in /private/tmp/source-mono-mac-4.2.0-branch/bockbuild-mono-4.2.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.2.1/external/referencesource/System.Core/System/Linq/Enumerable.cs:950
at XcodeBuilder.Project.EnsureFileReferenceExists (Bam.Core.TokenizedString path, EFileType type, Boolean explicitType, ESourceTree sourceTree) [0x00025] in /Users/mark/dev/BuildAMation/packages/XcodeBuilder/bam/Scripts/Project.cs:248
at XcodeBuilder.Target.EnsureBuildFileExists (Bam.Core.TokenizedString path, EFileType type) [0x0000b] in /Users/mark/dev/BuildAMation/packages/XcodeBuilder/bam/Scripts/Target.cs:176
at XcodeBuilder.Target.EnsureSourceBuildFileExists (Bam.Core.TokenizedString path, EFileType type) [0x0005b] in /Users/mark/dev/BuildAMation/packages/XcodeBuilder/bam/Scripts/Target.cs:200
at C.XcodeCompilation.C.ICompilationPolicy.Compile (C.ObjectFile sender, Bam.Core.ExecutionContext context, Bam.Core.TokenizedString objectFilePath, Bam.Core.Module source) [0x0009f] in /Users/mark/dev/BuildAMation/packages/C/bam/XcodeBuilder/XcodeCompilation.cs:69
at C.ObjectFile.ExecuteInternal (Bam.Core.ExecutionContext context) [0x00023] in /Users/mark/dev/BuildAMation/packages/C/bam/Scripts/ObjectFile.cs:128
at Bam.Core.Module.Bam.Core.IModuleExecution.Execute (Bam.Core.ExecutionContext context) <0x32b64f0 + 0x00143> in :0
at Bam.Core.Executor+c__AnonStorey2.<>m__0 () <0x32b5470 + 0x00266> in :0
at System.Threading.Tasks.Task.InnerInvoke () [0x00012] in /private/tmp/source-mono-mac-4.2.0-branch/bockbuild-mono-4.2.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.2.1/external/referencesource/mscorlib/system/threading/Tasks/Task.cs:2901
at System.Threading.Tasks.Task.Execute () [0x00016] in /private/tmp/source-mono-mac-4.2.0-branch/bockbuild-mono-4.2.0-branch/profiles/mono-mac-xamarin/build-root/mono-4.2.1/external/referencesource/mscorlib/system/threading/Tasks/Task.cs:2523