问"TypeError:无法读取属性'$router‘of null“中的

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"TypeError: Cannot read property '$router' of null"

<v-btn @click="this.$router.push({path: '/newpage'})">Next Page</v-btn>

import Vue from 'vue';
import Router from 'vue-router';
import NewPage from '../components/NewPage.vue';

Vue.use(Router)

export default new Router({
    base: process.eventNames.BASE_URL,
    routes: [
         {
            path: '/newpage',
            name: 'newpage',
            component: NewPage
        } 
    ]
})

<v-btn @click="NewPage">Next Page</v-btn>
.....
NewPage ( ) {
        this.$router.push({ path: '/newpage'});
}

"TypeError: Cannot read property 'push' of undefined"