Source

File: wp-includes/class-wp-taxonomy.php

public function get_rest_controller() {
		if ( ! $this->show_in_rest ) {
			return null;
		}

		$class = $this->rest_controller_class ? $this->rest_controller_class : WP_REST_Terms_Controller::class;

		if ( ! class_exists( $class ) ) {
			return null;
		}

		if ( ! is_subclass_of( $class, WP_REST_Controller::class ) ) {
			return null;
		}

		if ( ! $this->rest_controller ) {
			$this->rest_controller = new $class( $this->name );
		}

		if ( ! ( $this->rest_controller instanceof $class ) ) {
			return null;
		}

		return $this->rest_controller;
	}