fix: clear dropdown div before rendering content (#5429)

core/field_dropdown.js

@@ -315,6 +315,9 @@ FieldDropdown.prototype.showEditor_ = function(opt_e) {
   } else {
     this.menu_.openingCoords = null;
   }
+
+  // Remove any pre-existing elements in the dropdown.
+  Blockly.DropDownDiv.clearContent();
   // Element gets created in render.
   this.menu_.render(DropDownDiv.getContentDiv());
   const menuElement = /** @type {!Element} */ (this.menu_.getElement());