--- a/browser/components/customizableui/src/CustomizableUI.jsm
+++ b/browser/components/customizableui/src/CustomizableUI.jsm
@@ -1199,16 +1199,21 @@ let CustomizableUIInternal = {
         lastPopup = aNode.localName == "menupopup" ? aNode : lastPopup;
         aNode = aNode.parentNode;
       }
       return lastPopup;
     }
 
     let target = aEvent.originalTarget;
     let panel = this._getPanelForNode(aEvent.currentTarget);
+    // This can happen in e.g. customize mode. If there's no panel,
+    // there's clearly nothing for us to close; pretend we're interactive.
+    if (!panel) {
+      return true;
+    }
     // We keep track of:
     // whether we're in an input container (text field)
     let inInput = false;
     // whether we're in a popup/context menu
     let inMenu = false;
     // whether we're in a toolbarbutton/toolbaritem
     let inItem = false;
     // whether the current menuitem has a valid closemenu attribute