fix android issue

Author: zhuxingwei

com.unity.uiwidgets/Runtime/engine/AndroidPlatformUtil.cs

@@ -17,6 +17,21 @@ public static class UIWidgetsAndroidConfiguration{
 
         internal delegate string UnpackFileCallback(string file);
 
+        internal static bool FileExists(string file)
+        {
+            var path = "jar:file://" + Application.dataPath + "!/assets/" + file;
+            using (var unpackerWWW = UnityWebRequest.Get(path)) {
+                unpackerWWW.SendWebRequest();
+                    while (!unpackerWWW.isDone) {
+                    } // This will block in the webplayer.
+
+                    if (unpackerWWW.isNetworkError || unpackerWWW.isHttpError) {
+                        return false;
+                    }
+            }
+            return true;
+        }
+
         [MonoPInvokeCallback(typeof(UnpackFileCallback))]
         internal static string unpackFile(string file) {
             var dir = Application.temporaryCachePath + "/";

com.unity.uiwidgets/Runtime/engine/UIWidgetsPanel.cs

@@ -76,7 +76,11 @@ public object fontsToObject() {
                     if (setting.Value.fonts[j].asset.Length > 0) {
                         var assetPath = setting.Value.fonts[j].asset;
                         var assetAbsolutePath = Path.Combine(Application.streamingAssetsPath, assetPath);
+#if !UNITY_EDITOR && UNITY_ANDROID
+                        if (!AndroidPlatformUtil.FileExists(assetPath)) {
+#else
                         if (!File.Exists(assetAbsolutePath)) {
+#endif
                             Debug.LogError($"The font asset (family: \"{setting.Key}\", path: \"{assetPath}\") is not found");
                         }
                         else {