Audio Audio Import Settings Mixed Up When Stopping the audio In Inspector

hi. i'm using unity 2018.4.1. there is a bug with audio import setting. could you please test it and then report that bug? bacause i don't know how to do that. i tested it on two different machines.
the problem is if you import an audio(in my case an mp3).
then in the import settings inspector of that file, if i click on play icon, it will play, but if i click it again(to Stop) the inspector will mix up and console shows me repeatedly this error:

NullReferenceException
UnityEditor.AudioClipInspector.get_playing () (at <efa5608ae90b47908ea24a4ae89706d2>:0)
UnityEditor.AudioClipInspector.OnPreviewSettings () (at C:/buildslave/unity/build/Editor/Mono/Inspector/AudioClipInspector.cs:135)
UnityEditor.Editor.OnPreviewSettings () (at C:/buildslave/unity/build/Editor/Mono/Inspector/Editor.cs:931)
UnityEditor.InspectorWindow.DrawPreviewAndLabels () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:995)
UnityEditor.InspectorWindow.OnGUI () (at C:/buildslave/unity/build/Editor/Mono/Inspector/InspectorWindow.cs:689)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <d7ac571ca2d04b2f981d0d886fa067cf>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <d7ac571ca2d04b2f981d0d886fa067cf>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <d7ac571ca2d04b2f981d0d886fa067cf>:0)
UnityEditor.HostView.Invoke (System.String methodName, System.Object obj) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:342)
UnityEditor.HostView.Invoke (System.String methodName) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:336)
UnityEditor.HostView.InvokeOnGUI (UnityEngine.Rect onGUIPosition, UnityEngine.Rect viewRect) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:310)
UnityEditor.DockArea.DrawView (UnityEngine.Rect viewRect, UnityEngine.Rect dockAreaRect, System.Boolean customBorder, System.Boolean floatingWindow, System.Boolean isBottomTab) (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:361)
UnityEditor.DockArea.OldOnGUI () (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:320)
UnityEngine.Experimental.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 worldTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout) (at C:/buildslave/unity/build/Modules/UIElements/IMGUIContainer.cs:244)
UnityEngine.GUIUtilityrocessEvent(Int32, IntPtr)