----ONLY ON OSX-------

Error when canceling Signal creation dialog

repro:
- Add Signal Emitter to a Timeline track
- open the Emit Signal pull-down, and select Create Signal...
- click Cancel in the save dialog window...
error is generated in the console:

InvalidOperationException: Stack empty.
System.Collections.Generic.Stack`1[T].ThrowForEmptyStack () (at <7483795e405e497399bf272766b56895>:0)
System.Collections.Generic.Stack`1[T].Pop () (at <7483795e405e497399bf272766b56895>:0)
UnityEditor.EditorGUI.EndChangeCheck () (at /Users/builduser/buildslave/unity/build/Editor/Mono/EditorGUI.cs:352)
UnityEditor.EditorGUI+ChangeCheckScope.CloseScope () (at /Users/builduser/buildslave/unity/build/Editor/Mono/EditorGUI.cs:330)
UnityEngine.GUI+Scope.Dispose () (at /Users/builduser/buildslave/unity/build/Modules/IMGUI/GUI.cs:1713)
UnityEditor.Timeline.Signals.SignalEmitterInspector.OnInspectorGUI () (at /Applications/Unity/Hub/Editor/2019.1.0b9/Unity.app/Contents/Resources/PackageManager/BuiltInPackages/com.unity.timeline/Editor/Signals/SignalEmitterInspector.cs:99)
UnityEditor.UIElements.InspectorElement+<CreateIMGUIInspectorFromEditor>c__AnonStorey1.<>m__0 () (at /Users/builduser/buildslave/unity/build/Editor/Mono/Inspector/InspectorElement.cs:441)
UnityEditor.PopupCallbackInfo:SetEnumValueDelegate(Object, String[], Int32)

This is a 2019.1 Timeline feature and this bug is repro in Version 2019.1.0b9 (f1481f10a935) & 2019.2