fix: Fix NullReferenceException when disabling a flag on SignalingManager inspector (#935)

* fix

* fix

Author: karasusan

com.unity.renderstreaming/Editor/SignalingManagerEditor.cs

@@ -33,6 +33,8 @@ internal class SignalingManagerEditor : UnityEditor.Editor
 
         private void OnEnable()
         {
+            EditorApplication.projectChanged += OnProjectChanged;
+
             m_UseDefault = serializedObject.FindProperty(SignalingManager.UseDefaultPropertyName);
             m_SignalingSettingsObject = serializedObject.FindProperty(SignalingManager.SignalingSettingsObjectPropertyName);
             m_SignalingSettings = serializedObject.FindProperty(SignalingManager.SignalingSettingsPropertyName);
@@ -41,6 +43,11 @@ private void OnEnable()
             m_EvaluateCommandlineArguments = serializedObject.FindProperty(SignalingManager.EvaluateCommandlineArgumentsPropertyName);
         }
 
+        private void OnDisable()
+        {
+            EditorApplication.projectChanged -= OnProjectChanged;
+        }
+
         public override VisualElement CreateInspectorGUI()
         {
             root = new VisualElement();
@@ -72,8 +79,6 @@ public override VisualElement CreateInspectorGUI()
             root.Add(new PropertyField(m_RunOnAwake, "Run On Awake"));
             root.Add(new PropertyField(m_EvaluateCommandlineArguments, "Evaluate Commandline Arguments"));
 
-            EditorApplication.projectChanged += OnProjectChanged;
-
             // Disable UI when running in Playmode
             EditorApplication.playModeStateChanged += OnPlayModeStateChanged;
             if (EditorApplication.isPlaying)
@@ -146,6 +151,8 @@ private void OnPlayModeStateChanged(PlayModeStateChange e)
 
         private void OnProjectChanged()
         {
+            if (root == null)
+                return;
             var paths = GetAvailableSignalingSettingsPath();
 
             // Force to use default settings if there are no available settings in project folder.
@@ -168,7 +175,6 @@ private void OnProjectChanged()
             }
             signalingSettingsPopupField.choices = availableObjects.ToList();
             signalingSettingsPopupField.index = defaultIndex;
-
         }
 
         private void OnClickedOpenProjectSettingsButton()