Log In user programmatically in Flutter WebView - flutter

Question

I am using webview_flutter.
I currently have a native login page in my app that takes in a user's name and password. I am trying to find a way to automatically fill in the username and password on the specific page in webview and submit the login request.
Tried doing this
webViewController.evaluateJavascript(
'''
var email = document.getElementById("CustomerEmail");
var password = document.getElementById("CustomerPassword");
email.value = "user#gmail.com";
password.value = "test123";
'''
);
but got the following errors:
Error Domain=WKErrorDomain Code=4 "A JavaScript exception occurred" UserInfo={WKJavaScriptExceptionLineNumber=3, WKJavaScriptExceptionMessage=TypeError: null is not an object (evaluating 'email.value = "user#gmail.com"'), WKJavaScriptExceptionColumnNumber=11, WKJavaScriptExceptionSourceURL=undefined, NSLocalizedDescription=A JavaScript exception occurred})
Greatly appreciate any help on this. Thank you.

Answer 1

The code works, I put it in the wrong place.
The code should be placed under onPageFinished. I had it under onWebViewCreated previously.
The following code works correctly.
WebView(
initialUrl: widget.url,
onPageFinished: (_) {
setState(() {
print("loggedin " + loggedIn.toString());
if(loggedIn == false) {
loggedIn = true;
_controller.future
.then((value) =>
value.evaluateJavascript('''
var email = document.getElementById("CustomerEmail");
var password = document.getElementById("CustomerPassword");
email.value = "user#gmail.com";
password.value = "test123";
document.getElementById('customer_login').submit();
'''));
}
});
},
javascriptMode: JavascriptMode.unrestricted,
onWebViewCreated: (WebViewController webViewController) {
_controller.complete(webViewController);
},
),