fix(android): onMessage exception on fallback path (react-native-webview#3741)

Author: Kudo

android/src/main/java/com/reactnativecommunity/webview/RNCWebView.java

@@ -449,7 +449,8 @@ protected class RNCWebViewBridge {
         @JavascriptInterface
         public void postMessage(String message) {
             if (mWebView.getMessagingEnabled()) {
-                mWebView.onMessage(message, mWebView.getUrl());
+                // Post to main thread because `mWebView.getUrl()` requires to be executed on main.
+                mWebView.post(() -> mWebView.onMessage(message, mWebView.getUrl()));
             } else {
                 FLog.w(TAG, "ReactNativeWebView.postMessage method was called but messaging is disabled. Pass an onMessage handler to the WebView.");
             }