feat(NetworkManager): Clear Spawnable Prefabs button (#3619)

Assets/Mirror/Editor/NetworkManagerEditor.cs

@@ -50,6 +50,13 @@ public override void OnInspectorGUI()
             {
                 ScanForNetworkIdentities();
             }
+
+            // clicking the Populate button in a large project can add hundreds of entries.
+            // have a clear button in case that wasn't intended.
+            if (GUILayout.Button("Clear Spawnable Prefabs"))
+            {
+                ClearNetworkIdentities();
+            }
         }
 
         void ScanForNetworkIdentities()
@@ -117,6 +124,20 @@ void ScanForNetworkIdentities()
             }
         }
 
+        void ClearNetworkIdentities()
+        {
+            // RecordObject is needed for "*" to show up in Scene.
+            // however, this only saves List.Count without the entries.
+            Undo.RecordObject(networkManager, "NetworkManager: cleared prefabs");
+
+            // add the entries
+            networkManager.spawnPrefabs.Clear();
+
+            // SetDirty is required to save the individual entries properly.
+            EditorUtility.SetDirty(target);
+
+        }
+
         static void DrawHeader(Rect headerRect)
         {
             GUI.Label(headerRect, "Registered Spawnable Prefabs:");