title: Error in v-on handler: "TypeError: this is null"
body: <p>I'm having trouble when trying to add 1 when clicking a button in Vue</p>
  
  <p>I have created a function sumar, I call it in the v-on assigned to the button, created the "contador"  but I have an error. It says this is null; I don't know why, as I have initialized the variable contador to 0 in the props.</p>
  
  <pre><code>&lt;template&gt;
  
  &lt;section  class="articulos b-flex b-flex-wrap b-flex-center b-flex-center-horizontal"&gt;
  
                  &lt;p&gt;{{ contador }}&lt;/p&gt;
  
  
  
          &lt;article v-for="imagen in imagenes" :key="imagen.id"&gt;
  
              &lt;figure class="contenedor-articulo"&gt;
  
                      &lt;img :src="require(`@/assets/img/${imagen.url}`)" /&gt; 
  
                          &lt;figcaption&gt;
  
                              &lt;h3&gt;{{imagen.nombre}}&lt;/h3&gt;
  
  
  
                              &lt;p&gt;+ 0,50 €&lt;/p&gt;
  
                              &lt;button v-on:click="sumar"&gt;&lt;i class="fas fa-plus-circle"&gt;&lt;/i&gt;&lt;/button&gt;
  
                          &lt;/figcaption&gt;
  
              &lt;/figure&gt;
  
          &lt;/article&gt;
  
  
      &lt;/section&gt;
  
  
  &lt;/template&gt;
  
  &lt;script&gt;
  
      export default {
    name: 'Articulos',
  
    props: {
  
        contador: 0
    },
  
    data() {
  
        return {
            imagenes: [
  
                  {id:1, url:"apeteat_2019_ensaladaquinoachicken.jpg", nombre: "Ensalada quinoa chicken"},
                  {id:2, url:"apeteat_2017__ensaladilla_rusa.jpg", nombre: "Ensaladilla rusa"},
                  {id:3,url:"apeteat_2018_nigirimix.jpg", nombre: "Niguiri mix"},
                  {id:4, url:"apeteat_2019_wrapcesar.jpg", nombre: "Wrap cesar"},
                  {id:5, url:"apeteat_2019_ensaladaquinoachicken.jpg", nombre: "Ensalada quinoa chicken"},  
                  {id:6, url:"apeteat_2018_nigirimix.jpg", nombre: "Niguiri mix"}
  
            ],
  
            sumar: function () {
          this.contador++ 
                  }
  
  
              }
          }
  
  }
  &lt;/script&gt;
  
  
  </code></pre>
  
  <p>I have the following error message:</p>
  
  <p>Error in v-on handler: "TypeError: this is null"</p>