fix: $apollo null error if smar obj is destroyed but has throttled/debounced calls

packages/vue-apollo-option/src/smart-apollo.js

@@ -98,6 +98,7 @@ export default class SmartApollo {
         const queryCb = this.initialOptions[prop].bind(this.vm)
         this.options[prop] = queryCb()
         let cb = query => {
+          if (this._destroyed) return
           this.options[prop] = query
           this.refresh()
         }

packages/vue-apollo-option/src/smart-query.js

@@ -94,6 +94,8 @@ export default class SmartQuery extends SmartApollo {
   }
 
   executeApollo (variables) {
+    if (this._destroyed) return
+
     const variablesJson = JSON.stringify(variables)
 
     if (this.sub) {

packages/vue-apollo-option/src/smart-subscription.js

@@ -28,6 +28,8 @@ export default class SmartSubscription extends SmartApollo {
   }
 
   executeApollo (variables) {
+    if (this._destroyed) return
+
     const variablesJson = JSON.stringify(variables)
     if (this.sub) {
       // do nothing if subscription is already running using exactly the same variables