fix: #3262 allow running a mirror project from edit prefab mode again (…

…#3924)

Co-authored-by: mischa <info@noobtuts.com>

Assets/Mirror/Editor/NetworkScenePostProcess.cs

@@ -55,19 +55,28 @@ public static void OnPostProcessScene()
                     else
                     {
                         // there are two cases where sceneId == 0:
-                        // * if we have a prefab open in the prefab scene
-                        // * if an unopened scene needs resaving
-                        // show a proper error message in both cases so the user
-                        // knows what to do.
+                        // if we have a prefab open in the prefab scene
                         string path = identity.gameObject.scene.path;
                         if (string.IsNullOrWhiteSpace(path))
+                        {
+                            // pressing play while in prefab edit mode used to freeze/crash Unity 2019.
+                            // this seems fine now so we don't need to stop the editor anymore.
+#if UNITY_2020_3_OR_NEWER
+                            Debug.LogWarning($"{identity.name} was open in Prefab Edit Mode while launching with Mirror. If this causes issues, please let us know.");
+#else
                             Debug.LogError($"{identity.name} is currently open in Prefab Edit Mode. Please open the actual scene before launching Mirror.");
+                            EditorApplication.isPlaying = false;
+#endif
+                        }
+                        // if an unopened scene needs resaving
                         else
-                            Debug.LogError($"Scene {path} needs to be opened and resaved, because the scene object {identity.name} has no valid sceneId yet.");
+                        {
 
-                        // either way we shouldn't continue. nothing good will
-                        // happen when trying to launch with invalid sceneIds.
-                        EditorApplication.isPlaying = false;
+                            // nothing good will happen when trying to launch with invalid sceneIds.
+                            // show an error and stop playing immediately.
+                            Debug.LogError($"Scene {path} needs to be opened and resaved, because the scene object {identity.name} has no valid sceneId yet.");
+                            EditorApplication.isPlaying = false;
+                        }
                     }
                 }
             }