Return value of Symfony\...\SelectorNode::getPseudoElement() must be of the type string, null returned

I have a problem like this problem:

When I put an email in reset oassword page, it shows:

Type error: Return value of Symfony\Component\CssSelector\Node\SelectorNode::getPseudoElement() must be of the type string, null returned in SelectorNode.php (line 42)

A person said here this error is for php version ( php v7.0 not supports nullable types) and he suggested change the php version But I can't change it, I must use from php v7.0 in this script, how I must do for fix this error?

via Chebli Mohamed