postMessage fix

windows/ReactNativeWebView/ReactWebViewManager.cpp

             else if (commandId == L"injectJavaScript") {
                 webView.InvokeScriptAsync(L"eval", { winrt::to_hstring(commandArgs[0].AsString()) });
             } else if(commandId == L"postMessage") {
-                if (auto reactWebView = content.try_as<ReactNativeWebView::ReactWebView>()) {
+                if (auto reactWebView = view.try_as<ReactNativeWebView::ReactWebView>()) {
                     reactWebView.PostMessage(winrt::to_hstring(commandArgs[0].AsString()));
                 }
             }