Bug report, this error message pops up when I was trying to save a changed asset through my scripts (AssetDatabase.SaveAssets(). These related assets are neither assigned in the Addressables Groups, nor marked as Addressables.
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.AddressableAssets.Settings.GroupSchemas.BundledAssetGroupSchema.get_HostingServicesContentRoot () (at Library/PackageCache/com.unity.addressables@1.13.1/Editor/Settings/GroupSchemas/BundledAssetGroupSchema.cs:249)
UnityEditor.AddressableAssets.HostingServices.HostingServicesManager.GetAllContentRoots () (at Library/PackageCache/com.unity.addressables@1.13.1/Editor/HostingServices/HostingServicesManager.cs:430)
UnityEditor.AddressableAssets.HostingServices.HostingServicesManager.ConfigureAllHostingServices () (at Library/PackageCache/com.unity.addressables@1.13.1/Editor/HostingServices/HostingServicesManager.cs:407)
UnityEditor.AddressableAssets.HostingServices.HostingServicesManager.OnSettingsModification (UnityEditor.AddressableAssets.Settings.AddressableAssetSettings s, UnityEditor.AddressableAssets.Settings.AddressableAssetSettings+ModificationEvent evt, System.Object obj) (at Library/PackageCache/com.unity.addressables@1.13.1/Editor/HostingServices/HostingServicesManager.cs:400)
UnityEditor.AddressableAssets.Settings.AddressableAssetSettings.SetDirty (UnityEditor.AddressableAssets.Settings.AddressableAssetSettings+ModificationEvent modificationEvent, System.Object eventData, System.Boolean postEvent, System.Boolean settingsModified) (at Library/PackageCache/com.unity.addressables@1.13.1/Editor/Settings/AddressableAssetSettings.cs:1214)
UnityEditor.AddressableAssets.Settings.AddressableAssetSettings.OnPostprocessAllAssets (System.String[ ] importedAssets, System.String[ ] deletedAssets, System.String[ ] movedAssets, System.String[ ] movedFromAssetPaths) (at Library/PackageCache/com.unity.addressables@1.13.1/Editor/Settings/AddressableAssetSettings.cs:1686)
UnityEditor.AddressableAssets.Settings.AddressablesAssetPostProcessor.OnPostprocessAllAssets (System.String[ ] importedAssets, System.String[ ] deletedAssets, System.String[ ] movedAssets, System.String[ ] movedFromAssetPaths) (at Library/PackageCache/com.unity.addressables@1.13.1/Editor/Settings/AddressableAssetPostProcessor.cs:39)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[ ] parameters, System.Globalization.CultureInfo culture) (at :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 :0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[ ] parameters) (at :0)
UnityEditor.AssetPostprocessingInternal.InvokeMethod (System.Reflection.MethodInfo method, System.Object[ ] args) (at :0)
UnityEditor.AssetPostprocessingInternal.PostprocessAllAssets (System.String[ ] importedAssets, System.String[ ] addedAssets, System.String[ ] deletedAssets, System.String[ ] movedAssets, System.String[ ] movedFromPathAssets) (at :0)
UnityEditor.AssetDatabase:SaveAssets()
PanoramaVideoDemo.CustomBuildHelper_UnitySetting:SetXRLoader(BuildTargetGroup, String) (at Assets/AppBuildScripts/Editor/CustomBuildHelper_UnitySetting.cs:353)
PanoramaVideoDemo.CustomBuildMenu:postSwitch() (at Assets/AppBuildScripts/Editor/CustomBuildMenu.cs:41)
PanoramaVideoDemo.CustomBuildMenu:SwitchToOculusMobileVR() (at Assets/AppBuildScripts/Editor/CustomBuildMenu.cs:162)