This worked fine in 3.4.
When doing this:
featLayer.SetDefinitionQuery(null);
I get this error (in Visual Studio and in my code's error log), and Arc exits without an error message.
:System.InvalidOperationException: A new active definition query could not be added.
at ArcGIS.Desktop.Internal.Mapping.Operations.SetDefinitionQueryOperation.<Do>b__10_0()
at ArcGIS.Desktop.Internal.Mapping.Utilities.MakeBasicServiceCall(Action serviceCall)
at ArcGIS.Desktop.Internal.Mapping.Operations.SetDefinitionQueryOperation.Do()
at ArcGIS.Desktop.Framework.OperationManager.Do(Operation operation)
at ArcGIS.Desktop.Mapping.BasicFeatureLayer.SetDefinitionQuery(String whereClause)
at NetMapPro.classLayers.<>c__DisplayClass27_1.<<drawReach>b__0>d.MoveNext()
--- End of stack trace from previous location ---
at NetMapPro.classLayers.drawReach(Layer lReach, String Attribute, Int32 Breaknum, String def)
I found another reference to this error on this page:
https://www.autodesk.com/support/technical/article/caas/sfdcarticles/sfdcarticles/Could-not-initialize-map-definition-query.html