Vuforia SDK Version: 9.8.8 -

Description with steps to reproduce: Windows 10, 64-bit
MSVC 2019 16.9.5
Vuforia 9.8.8
Unity 2020.3.8f1 LTS

Steps:
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1. In a scene with Vuforia in it (see note below), press play in the Unity editor.
2. While the editor is in play mode, edit a C# "script" in VS.
3. Switch focus back to the Unity editor. It will recompile the scripts.
4. Stop playback.
5. Start playback again.

Expected Behavior:
---

No issues.

Actual Behavior:
---

Note: Not sure what setup causes this, but in my scene there is a single ReferenceImageTarget and Vuforia is set up to use WEBCAM play mode.

As soon as the scripts are recompiled (step 3), the following error appears in the console:

NullReferenceException: Object reference not set to an instance of an object
Vuforia.VideoBackgroundBehaviour.OnDestroy () (at :0)

On stopping playback (step 4), nothing of interest happens. However, when restarting playback (step 5), one of two things happens (not sure what determines the difference):

About 50% of the time the game screen is replaced with a "Vuforia Initialization Error" ("INIT ERROR: Failed to initialize Vuforia Engine"). When that happens, sometimes you can try again, sometimes not, but the editor becomes generally unstable. About 50% of the time, though, Unity just crashes right away (contact me for logs, not available right now).

Log for Vuforia Initialization Error:

AR:ERROR: 2021-05-17 14:22:14: Vuforia is already initialized. You need to deinitialize before initializing again.
Vuforia initialization failed
UnityEngine.Debug:LogError (object)
Vuforia.VuforiaRuntime:CheckInitStatus (Vuforia.VuforiaUnity/InitError)
Vuforia.VuforiaRuntime:InitInternal ()
Vuforia.VuforiaRuntime:InitVuforia ()
Vuforia.VuforiaRuntime:VuforiaInitialization ()

Vuforia Engine initialization failed: INIT_ERROR
INIT ERROR
Failed to initialize Vuforia Engine.
UnityEngine.Debug:LogError (object)
DefaultInitializationErrorHandlerInternal:SetErrorCode (Vuforia.VuforiaUnity/InitError)
DefaultInitializationErrorHandlerInternal:OnVuforiaInitializationError (Vuforia.VuforiaUnity/InitError)
System.Reflection.MonoMethod:Invoke (object,System.Reflection.BindingFlags,System.Reflection.Binder,object[],System.Globalization.CultureInfo)
System.Reflection.MethodBase:Invoke (object,object[])
System.Delegate:DynamicInvokeImpl (object[])
System.MulticastDelegate:DynamicInvokeImpl (object[])
System.Delegate:DynamicInvoke (object[])
Vuforia.DelegateHelper:InvokeDelegate (System.Delegate,object[])
Vuforia.DelegateHelper:InvokeWithExceptionHandling (System.Action`1,Vuforia.VuforiaUnity/InitError)
Vuforia.VuforiaRuntime:CheckInitStatus (Vuforia.VuforiaUnity/InitError)
Vuforia.VuforiaRuntime:InitInternal ()
Vuforia.VuforiaRuntime:InitVuforia ()
Vuforia.VuforiaRuntime:VuforiaInitialization ()

NullReferenceException: Object reference not set to an instance of an object
at UnityEditor.PropertyHandler.IsArrayReorderable (UnityEditor.SerializedProperty property) [0x0005b] in :0
at UnityEditor.PropertyHandler.UseReorderabelListControl (UnityEditor.SerializedProperty property) [0x00008] in :0
at UnityEditor.PropertyHandler.GetHeight (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) [0x00059] in :0
at UnityEditor.EditorGUI.GetPropertyHeightInternal (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) [0x00007] in :0
at UnityEditor.EditorGUI.GetPropertyHeight (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) [0x00001] in :0
at UnityEditor.Editor.CanBeExpandedViaAFoldoutWithoutUpdate () [0x00036] in :0
at UnityEditor.UIElements.EditorElement.UpdateInspectorVisibility () [0x00007] in :0
at UnityEditor.UIElements.EditorElement.Init () [0x000d2] in :0
at UnityEditor.UIElements.EditorElement..ctor (System.Int32 editorIndex, UnityEditor.IPropertyView iw, System.Boolean isCulled) [0x00034] in :0
at UnityEditor.EditorUIServiceImpl.CreateEditorElement (System.Int32 editorIndex, UnityEditor.IPropertyView iw, System.String title) [0x00000] in :0
at UnityEditor.PropertyEditor.DrawEditors (UnityEditor.Editor[] editors) [0x002e4] in :0
at UnityEditor.PropertyEditor.RebuildContentsContainers () [0x00119] in :0
at UnityEditor.InspectorWindow.RedrawFromNative () [0x00012] in :0

NullReferenceException: Object reference not set to an instance of an object
at UnityEditor.PropertyHandler.IsArrayReorderable (UnityEditor.SerializedProperty property) [0x0005b] in :0
at UnityEditor.PropertyHandler.UseReorderabelListControl (UnityEditor.SerializedProperty property) [0x00008] in :0
at UnityEditor.PropertyHandler.GetHeight (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) [0x00059] in :0
at UnityEditor.EditorGUI.GetPropertyHeightInternal (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) [0x00007] in :0
at UnityEditor.EditorGUI.GetPropertyHeight (UnityEditor.SerializedProperty property, UnityEngine.GUIContent label, System.Boolean includeChildren) [0x00001] in :0
at UnityEditor.Editor.CanBeExpandedViaAFoldoutWithoutUpdate () [0x00036] in :0
at UnityEditor.UIElements.EditorElement.UpdateInspectorVisibility () [0x00007] in :0
at UnityEditor.UIElements.EditorElement.Init () [0x000d2] in :0
at UnityEditor.UIElements.EditorElement..ctor (System.Int32 editorIndex, UnityEditor.IPropertyView iw, System.Boolean isCulled) [0x00034] in :0
at UnityEditor.EditorUIServiceImpl.CreateEditorElement (System.Int32 editorIndex, UnityEditor.IPropertyView iw, System.String title) [0x00000] in :0
at UnityEditor.PropertyEditor.DrawEditors (UnityEditor.Editor[] editors) [0x002e4] in :0
at UnityEditor.PropertyEditor.RebuildContentsContainers () [0x00119] in :0
at UnityEditor.InspectorWindow.RedrawFromNative () [0x00012] in :0

Vuforia Engine cannot be started before it is initialized.
Please disable Delayed Initializationin the Vuforia configuration or initialize Vuforia manually with the VuforiaRuntime-class.
UnityEngine.Debug:LogError (object)
Vuforia.VuforiaARController:Start ()
System.Reflection.MonoMethod:Invoke (object,System.Reflection.BindingFlags,System.Reflection.Binder,object[],System.Globalization.CultureInfo)
System.Reflection.MethodBase:Invoke (object,object[])
System.Delegate:DynamicInvokeImpl (object[])
System.MulticastDelegate:DynamicInvokeImpl (object[])
System.Delegate:DynamicInvoke (object[])
Vuforia.DelegateHelper:InvokeDelegate (System.Delegate,object[])
Vuforia.DelegateHelper:InvokeWithExceptionHandling (System.Action)
Vuforia.VuforiaBehaviour:Start () -

Development OS (Mac OS X, Windows, Linux): Windows 10 -

Mobile OS and Version: not applicable -

Mobile Device Manufacturer and Model name: not applicable -

Do the Vuforia Sample Applications show the same behavior?: Unknown