Popover: execute focus on instance only if it is a function

packages/popover/src/main.vue

@@ -90,7 +90,7 @@ export default {
         on(reference, 'focusin', () => {
           this.handleFocus();
           const instance = reference.__vue__;
-          if (instance && instance.focus) {
+          if (instance && typeof instance.focus === 'function') {
             instance.focus();
           }
         });