Laravel 5.7 Account Varification Email gives "Illegal offset type" Exception

Question

I implemented Laravel's Authentication. Everything is working fine except account verification. When I Signup/Regiser I receive Account Verification email with a verification link. When I click on provided link to verify my account I got following Exception

ErrorException (E_WARNING)
Illegal offset type
/var/www/html/myproject/vendor/laravel/framework/src/Illuminate/Auth/AuthManager.php

public function __construct($app)
{
    $this->app = $app;

    $this->userResolver = function ($guard = null) {
        return $this->guard($guard)->user();
    };
}

/**
 * Attempt to get the guard from the local cache.
 *
 * @param  string  $name
 * @return \Illuminate\Contracts\Auth\Guard|\Illuminate\Contracts\Auth\StatefulGuard
 */
public function guard($name = null)
{
    $name = $name ?: $this->getDefaultDriver();

    return $this->guards[$name] ?? $this->guards[$name] = $this->resolve($name);
}