Contact Form: avoid errors when saved feedback is not a WP_Post (#34129)

Fixes #34125

Saved feedback forms can be `WP_Post`, `array`, or `null`.
Let's not assume we'll always get a `WP_Post`, and be more defensive.

projects/packages/forms/changelog/fix-defensive-post-check-contact-form

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Avoid errors when a saved feedback form does not have the expected WP_Post format.

projects/packages/forms/src/contact-form/class-contact-form.php

@@ -526,7 +526,7 @@ public static function get_compiled_form( $feedback_id, $form ) {
 					}
 				} else {
 					// The feedback content is stored as the first "half" of post_content
-					$value         = $feedback->post_content;
+					$value         = is_a( $feedback, '\WP_Post' ) ? $feedback->post_content : '';
 					list( $value ) = explode( '<!--more-->', $value );
 					$value         = trim( $value );
 				}
@@ -619,7 +619,7 @@ public static function get_compiled_form_for_email( $feedback_id, $form ) {
 					}
 				} else {
 					// The feedback content is stored as the first "half" of post_content
-					$value         = $feedback->post_content;
+					$value         = is_a( $feedback, '\WP_Post' ) ? $feedback->post_content : '';
 					list( $value ) = explode( '<!--more-->', $value );
 					$value         = trim( $value );
 				}

projects/plugins/jetpack/changelog/fix-defensive-post-check-contact-form

@@ -0,0 +1,4 @@
+Significance: patch
+Type: bugfix
+
+Contact Form: avoid errors when a saved submitted contact form is requested but does not exist anymore.

projects/plugins/jetpack/modules/contact-form/grunion-contact-form.php

@@ -3089,7 +3089,7 @@ public static function get_compiled_form( $feedback_id, $form ) {
 					}
 				} else {
 					// The feedback content is stored as the first "half" of post_content
-					$value         = $feedback->post_content;
+					$value         = is_a( $feedback, '\WP_Post' ) ? $feedback->post_content : '';
 					list( $value ) = explode( '<!--more-->', $value );
 					$value         = trim( $value );
 				}
@@ -3182,7 +3182,7 @@ public static function get_compiled_form_for_email( $feedback_id, $form ) {
 					}
 				} else {
 					// The feedback content is stored as the first "half" of post_content
-					$value         = $feedback->post_content;
+					$value         = is_a( $feedback, '\WP_Post' ) ? $feedback->post_content : '';
 					list( $value ) = explode( '<!--more-->', $value );
 					$value         = trim( $value );
 				}