Fix PHPStan issues.

src/Extension.php

@@ -616,7 +616,7 @@ public function rcp_membership_payment_profile_cancelled( $success, $gateway, $g
 	 * Subscription action links HTML.
 	 *
 	 * @param string         $actions        Formatted HTML links.
-	 * @param array          $links          Links.
+	 * @param array<string>  $links          Links.
 	 * @param int            $user_id        Current user ID.
 	 * @param RCP_Membership $rcp_membership Membership object.
 	 * @return string
@@ -664,7 +664,7 @@ public function maybe_update_membership_gateway( $post_id ) {
 			return;
 		}
 
-		$rcp_membership = \rcp_get_membership( $subscription->get_source_id() );
+		$rcp_membership = \rcp_get_membership( (int) $subscription->get_source_id() );
 
 		if ( false === $rcp_membership ) {
 			return;

src/Gateways/Gateway.php

@@ -47,7 +47,7 @@ public function init() {
 		// Set supported features based on gateway.
 		$gateway = $this->get_pronamic_gateway();
 
-		if ( null !== $gateway ) {
+		if ( null !== $gateway && null !== $this->payment_method ) {
 			$payment_method = $gateway->get_payment_method( $this->payment_method );
 
 			if ( null !== $payment_method && $payment_method->supports( 'recurring' ) ) {
@@ -260,6 +260,10 @@ public function fields() {
 			return '';
 		}
 
+		if ( null === $this->payment_method ) {
+			return '';
+		}
+
 		$payment_method = $gateway->get_payment_method( $this->payment_method );
 
 		if ( null === $payment_method ) {