mainWebView.setWebViewClient(new WebViewClient(){
@Override
public boolean shouldOverrideUrlLoading(WebView view, WebResourceRequest request) {
//網頁在webView中開啟
view.loadUrl(request.toString());
return true;
}
});

替換以下方法：

mainWebView.setWebViewClient(new WebViewClient(){
@Override
public boolean shouldOverrideUrlLoading
 
(WebView view, WebResourceRequest request) {
//網頁在webView中開啟
if(Build.VERSION.SDK_INT <=  Build.VERSION_CODES.LOLLIPOP){//安卓5.0的載入方法
view.loadUrl(request.toString());
}else {//5.0以上的載入方法
view.loadUrl(request.getUrl().toString());
}
return true;
}
});

解決 ok