Value of type null is not callable

Question

I got the following error:

Value of type null is not callable

When trying to delete an activity,the function is called by:

$assignment->delete_instance();

The $assignment is not null i double checked, but a valid assignment object, i don't know how to solve this issue, please help in this question, thank you in advance

Edit1:

"It sounds like you reused a method name as a variable" - this is true because its definied like:

function remove_course_contents($courseid, $showfeedback = true, array $options = null) {
$modname = 'assign';
$moddelete = $modname .'_delete_instance'; 
                    if (function_exists($moddelete)) {
                    // This purges all module data in related tables, extra user prefs, settings, etc.
                    $moddelete($cm->modinstance);
                } else {
                    // NOTE: we should not allow installation of modules with missing delete support!
                    debugging("Defective module '$modname' detected when deleting course contents: missing function $moddelete()!");
                    $DB->delete_records($modname, array('id' => $cm->modinstance));
                }

so it's called like:

$moddelete($cm->modinstance);

this will call finally:

function assign_delete_instance($id) {
$assignment->delete_instance();//this is where the execution stopped
}

public function delete_instance() {
    global $DB;
    $result = true;

    foreach ($this->submissionplugins as $plugin) {
        if (!$plugin->delete_instance()) {
            throw new \moodle_exception($plugin->get_error());
            $result = false;
        }
    }
    foreach ($this->feedbackplugins as $plugin) {
        if (!$plugin->delete_instance()) {
            throw new \moodle_exception($plugin->get_error());
            $result = false;
        }
    }

    // Delete files associated with this assignment.
    $fs = get_file_storage();
    if (! $fs->delete_area_files($this->context->id) ) {
        $result = false;
    }

    $this->delete_all_overrides();

    // Delete_records will throw an exception if it fails - so no need for error checking here.
    $DB->delete_records('assign_submission', array('assignment' => $this->get_instance()->id));
    $DB->delete_records('assign_grades', array('assignment' => $this->get_instance()->id));
    $DB->delete_records('assign_plugin_config', array('assignment' => $this->get_instance()->id));
    $DB->delete_records('assign_user_flags', array('assignment' => $this->get_instance()->id));
    $DB->delete_records('assign_user_mapping', array('assignment' => $this->get_instance()->id));

    // Delete items from the gradebook.
    if (! $this->delete_grades()) {
        $result = false;
    }

    // Delete the instance.
    // We must delete the module record after we delete the grade item.
    $DB->delete_records('assign', array('id'=>$this->get_instance()->id));

    return $result;
}