client/html: fix upload error when pool input is disabled

client/js/controls/post_edit_sidebar_control.js

@@ -236,11 +236,13 @@ class PostEditSidebarControl extends events.EventTarget {
             );
         }
 
+        if (this._poolControl) {
             this._poolControl.addEventListener("change", (e) => {
                 this.dispatchEvent(new CustomEvent("change"));
                 this._syncExpanderTitles();
             });
         }
+    }
 
     _syncExpanderTitles() {
         this._notesExpander.title = `Notes (${this._post.notes.length})`;