Fix regression: Can't remove videos from local playlists

youtube/templates/base.html

@@ -142,9 +142,19 @@
                         window.setTimeout(removeMessage, removalDelay+300, messageBox);
                     }
                     // https://developer.mozilla.org/en-US/docs/Learn/HTML/Forms/Sending_forms_through_JavaScript
-                    function sendData(){
+                    function sendData(event){
+                        var clicked_button = document.activeElement;
+                        if(clicked_button === null || clicked_button.getAttribute('type') !== 'submit' || clicked_button.parentElement != event.target){
+                            console.log('ERROR: clicked_button not valid');
+                            return;
+                        }
+                        if(clicked_button.getAttribute('value') !== 'add'){
+                            return;     // video(s) are being removed from playlist, just let it refresh the page
+                        }
+                        event.preventDefault();
                         var XHR = new XMLHttpRequest();
                         var FD = new FormData(playlistAddForm);
+
                         // https://stackoverflow.com/questions/48322876/formdata-doesnt-include-value-of-buttons
                         FD.append('action', 'add');
 
@@ -168,10 +178,7 @@
                         XHR.send(FD);
                     }
 
-                    playlistAddForm.addEventListener('submit', function(event){
-                        event.preventDefault();
-                        sendData();
-                    });
+                    playlistAddForm.addEventListener('submit', sendData);
                 </script>
             {% endif %}
         </header>