mitt event bus with combobox usage in VueJS 3 - vue.js

Question

I have configured mitt as a global event bus in my main.js:
import { createApp } from "vue";
import App from './App.vue'
import mitt from "mitt";
const emitter = mitt();
const app = createApp(App);
app.config.globalProperties.emitter = emitter;
app.mount("#app");
And I'm publishing an event using the event-bus from the child component as follows:
<template>
<div class= "combobox">
<label for={{selector_name}}>
<p>{{ selector_title }}</p>
</label>
<select :name="selector_name" :id="selector_id" :disabled="disabled" #change="onChange">
<option v-for="(opt, index) in selector_options" :key="`opt ${index}`">
{{ opt }}
</option>
</select>
</div>
</template>
<script>
export default {
name: 'ComboBox',
data() {
return {
disabled: false,
};
},
props: {
selector_title: {
type: String,
required: true
},
selector_name: {
type: String,
required: true
},
selector_id: {
type: String,
default: "combobox"
},
selector_options: {
type: Array,
required: true
},
},
methods: {
onChange(event){
this.emitter.emit('lock-combobox', { id: this.selector_id });
},
},
}
</script>
Then I try to subscribe to the event in the parent component as follows:
<template>
<div id="controls" class="param">
<div class="control" id="combobox1">
<ComboBox
selector_title = "Pattern"
selector_name = "pattern"
:selector_options="['complex', 'simple']"
selector_id="combo1"
ref="combo1"
/>
</div>
<div class="control" id="combobox2">
<ComboBox
selector_title = "Hand"
selector_name = "pattern"
:selector_options="['Left', 'Right']"
selector_id="combo2"
ref="combo2"
/>
</div>
<div class="control" id="speedslider">
<SpeedSlider/>
</div>
<div class="control" id="startstop">
<StartStopButton/>
</div>
<div class = control id="emergency">
<Emergency/>
</div>
</div>
</template>
<script>
import ComboBox from '../components/ComboBox.vue';
export default {
name: "Controls",
components: {
ComboBox,
},
created() {
this.emitter.on('lock-combobox', (event) => {
this.$refs.event.id.disabled = true;
});
},
}
</script>
But whenever I change the option in the Combobox, I get an error:
Uncaught TypeError: undefined has no properties
Can you please tell me how can I solve that? thanks in advance.