8.X to 9.X upgrade error

hi we developed a package for 8.X and installed it at 9.X the same package but when I access the package dashboard page it is showing the following error can someone please help


Call to a member function get() on null

$_breadcrumb = $this->elementManager->get($this->getBreadcrumbElement(), [
            'breadcrumb' => $breadcrumb

If your package’s Dashboard page controllers define the on_start() method, make sure they also call parent::on_start()

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