fix(devtools): vue2 devtools timeout error (#209)

Author: DamianOsipiuk

src/vuejs/vueQueryPlugin.ts

@@ -77,6 +77,8 @@ export const VueQueryPlugin = {
 
     /* istanbul ignore next */
     if (isVue2) {
+      // Workaround for Vue2 calling mixin multiple times
+      let devtoolsRegistered = false;
       app.mixin({
         beforeCreate() {
           // HACK: taken from provide(): https://github.com/vuejs/composition-api/blob/master/src/apis/inject.ts#L30
@@ -87,10 +89,14 @@ export const VueQueryPlugin = {
               set: (v) => Object.assign(provideCache, v),
             });
           }
+
           this._provided[clientKey] = client;
 
-          if (process.env.NODE_ENV === "development") {
-            setupDevtools(this, client);
+          if (!devtoolsRegistered) {
+            if (process.env.NODE_ENV === "development") {
+              setupDevtools(this, client);
+              devtoolsRegistered = true;
+            }
           }
         },
       });