can not read property '_withtask' of undefined vue

I have a click event and I miss this error: can not read property '_withtask' of undefined.

My code is:

<button v-on:click="addNode"></button>

And as a method:

export default Vue.extend({ 
    methods: { 
        addNode: function () { 
            this.gridOptions.api.deselectAll() 
            var newNode = { 'channel': 'canal', } 
            this.collection = this.prepend(newNode, this.collection) 
            this.gridOptions.rowData = this.collection
        } 
    }
}

That basically what it does is add a new line. This new line is inserted, but this error does not allow me to continue interacting with the application. The problem is in the prepend, I tried with push but it gives me the same error ... What does this error mean?

Thank you very much!