Chart.combine sometimes results in "Collection was modified; enumeration operation may not execute."

System.InvalidOperationException: Collection was modified; enumeration operation may not execute.
   at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.KeyCollection.Enumerator.MoveNext()
   at <StartupCode$DynamicObj-FSC472>.$DynamicObj.GetProperties@81.GenerateNext(IEnumerable`1& next)
   at Microsoft.FSharp.Core.CompilerServices.GeneratedSequenceBase`1.MoveNextImpl()
   at Microsoft.FSharp.Collections.SeqModule.ToList[T](IEnumerable`1 source)
   at DynamicObj.DynamicObj.GetProperties(Boolean includeInstanceProperties)
   at DynamicObj.DynObj.combine(DynamicObj first, DynamicObj second)
   at DynamicObj.DynObj.combine(DynamicObj first, DynamicObj second)
   at DynamicObj.DynObj.combine(DynamicObj first, DynamicObj second)
   at Plotly.NET.GenericChart.combineLayouts@243(Layout first, Layout second)
   at Plotly.NET.GenericChart.combine@309.Invoke(GenericChart acc, GenericChart elem)

The sequences of data I'm feeding it are from immutable lists; so I don't think it's a problem with the way I'm calling Plotly.NET.

I can try again (with the same data) and usually it is successful. Seems to be a sporadic issue. Very odd.

Start with the stack trace through DynamicObj.GetProperties, DynObj.combine, GenericChart.combineLayouts, and GenericChart.combine; the issue names no source files or tests. Reproduce the intermittent exception while combining charts, then verify that chart combination no longer raises the collection-modified error and add regression coverage where the existing tests for this path are located.