public function process_upgrade_request() {

		$upgrade_id = isset( $_REQUEST['upgrade_id'] ) ? sanitize_key( $_REQUEST['upgrade_id'] ) : false;

		if ( ! $upgrade_id && ! $this->has_uncomplete_upgrades() ) {
			wp_send_json_error( array(
				'error' => __( 'A batch process ID must be present to continue.', 'popup-maker' ),
			) );
		}

		// Nonce.
		if ( ! check_ajax_referer( 'pum_upgrade_ajax_nonce', 'nonce' ) ) {
			wp_send_json_error( array(
				'error' => __( 'You do not have permission to initiate this request. Contact an administrator for more information.', 'popup-maker' ),
			) );
		}

		if ( ! $upgrade_id ) {
			$upgrade_id = $this->get_current_upgrade_id();
		}

		$step = ! empty( $_REQUEST['step'] ) ? absint( $_REQUEST['step'] ) : 1;

		/**
		 * Instantiate the upgrade class.
		 *
		 * @var PUM_Interface_Batch_Process|PUM_Interface_Batch_PrefetchProcess $upgrade
		 */
		$upgrade = $this->get_upgrade( $upgrade_id, $step );

		if ( $upgrade === false ) {
			wp_send_json_error( array(
				'error' => sprintf( __( '%s is an invalid batch process ID.', 'popup-maker' ), esc_html( $upgrade_id ) ),
			) );
		}

		/**
		 * Garbage collect any old temporary data in the case step is 1.
		 * Here to prevent case ajax passes step 1 without resetting process counts.
		 */
		$first_step = $step < 2;

		if ( $first_step ) {
			$upgrade->finish();
		}

		$using_prefetch = ( $upgrade instanceof PUM_Interface_Batch_PrefetchProcess );

		// Handle pre-fetching data.
		if ( $using_prefetch ) {
			// Initialize any data needed to process a step.
			$data = isset( $_REQUEST['form'] ) ? $_REQUEST['form'] : array();

			$upgrade->init( $data );
			$upgrade->pre_fetch();
		}

		/** @var int|string|WP_Error $step */
		$step = $upgrade->process_step();

		if ( ! is_wp_error( $step ) ) {
			$response_data = array(
				'step' => $step,
				'next' => null,
			);

			// Finish and set the status flag if done.
			if ( 'done' === $step ) {
				$response_data['done']    = true;
				$response_data['message'] = $upgrade->get_message( 'done' );

				// Once all calculations have finished, run cleanup.
				$upgrade->finish();

				// Set the upgrade complete.
				pum_set_upgrade_complete( $upgrade_id );

				if ( $this->has_uncomplete_upgrades() ) {
					// Since the other was complete return the next (now current) upgrade_id.
					$response_data['next'] = $this->get_current_upgrade_id();
				}
			} else {
				$response_data['done']       = false;
				$response_data['message']    = $first_step ? $upgrade->get_message( 'start' ) : '';
				$response_data['percentage'] = $upgrade->get_percentage_complete();
			}

			wp_send_json_success( $response_data );
		} else {
			wp_send_json_error( $step );
		}
	}