[unity] Fixed Editor error messages upon startup, most likely due to AssetDatabase not being fully initialized (Unity 2020.2). Closes #1779.

spine-unity/Assets/Spine/Editor/spine-unity/Editor/Utility/SpineEditorUtilities.cs

@@ -69,6 +69,7 @@ namespace Spine.Unity.Editor {
 		public static string editorPath = "";
 		public static string editorGUIPath = "";
 		public static bool initialized;
+		private static int numFramesToDelayInit = 1;
 		private static List<string> texturesWithoutMetaFile = new List<string>();
 
 		// Auto-import entry point for textures
@@ -96,11 +97,16 @@ namespace Spine.Unity.Editor {
 		}
 
 #region Initialization
+
 		static SpineEditorUtilities () {
-			Initialize();
+			EditorApplication.update += Initialize; // delayed so that AssetDatabase is ready.
 		}
 
 		static void Initialize () {
+			if (numFramesToDelayInit-- > 0)
+				return;
+			EditorApplication.update -= Initialize;
+
 			// Note: Preferences need to be loaded when changing play mode
 			// to initialize handle scale correctly.
 			#if !NEW_PREFERENCES_SETTINGS_PROVIDER