Fix crash when pressing ArrowUp the first time

js/src/dropdown.js

@@ -479,7 +479,7 @@ class Dropdown {
       return
     }
 
-    let index = items.indexOf(event.target) || 0
+    let index = items.indexOf(event.target)
 
     if (event.key === ARROW_UP_KEY && index > 0) { // Up
       index--
@@ -489,6 +489,9 @@ class Dropdown {
       index++
     }
 
+    // index is -1 if the first keydown is an ArrowUp
+    index = index === -1 ? 0 : index
+
     items[index].focus()
   }