fix: Fix NullReferenceException when disabling a flag on SignalingManager inspector

[karasusan]

When disabling the flag "Use Default Settings in Project Settings", throws an exception below.

NullReferenceException: SerializedObject of SerializedProperty has been Disposed.
UnityEditor.SerializedProperty.get_objectReferenceValue () (at /home/bokken/build/output/unity/unity/Editor/Mono/SerializedProperty.bindings.cs:1232)
Unity.RenderStreaming.Editor.SignalingManagerEditor.OnProjectChanged () (at ./Library/PackageCache/com.unity.renderstreaming@4e408551e619/Editor/SignalingManagerEditor.cs:159)
UnityEditor.EditorApplication.Internal_CallProjectHasChanged () (at /home/bokken/build/output/unity/unity/Editor/Mono/EditorApplication.cs:409)
UnityEditor.AssetDatabase:CopyAsset(String, String)
Unity.RenderStreaming.Editor.SignalingManagerEditor:CreateDefaultSignalingSettings() (at ./Library/PackageCache/com.unity.renderstreaming@4e408551e619/Editor/SignalingManagerEditor.cs:122)
Unity.RenderStreaming.Editor.SignalingManagerEditor:OnChangeUseDefault(SerializedPropertyChangeEvent) (at ./Library/PackageCache/com.unity.renderstreaming@4e408551e619/Editor/SignalingManagerEditor.cs:196)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&) (at /home/bokken/build/output/unity/unity/Modules/IMGUI/GUIUtility.cs:190)