feat: throw error when QueryClientProvider has not been used to set a query client (#1309)

Also updated docs

Author: JakeGinnivan

docs/src/pages/guides/migrating-to-react-query-3.md

@@ -22,6 +22,8 @@ This has some benefits:
 
 Use the `QueryClientProvider` component to connect a `QueryClient` to your application:
 
+**NOTE** There is no longer a default query cache, you must connect your application to a query provider manually
+
 ```js
 import { QueryClient, QueryClientProvider } from 'react-query'
 

src/react/QueryClientProvider.tsx

@@ -6,7 +6,13 @@ const QueryClientContext = React.createContext<QueryClient | undefined>(
   undefined
 )
 
-export const useQueryClient = () => React.useContext(QueryClientContext)!
+export const useQueryClient = () => {
+  const queryClient = React.useContext(QueryClientContext)
+  if (!queryClient) {
+    throw new Error('No QueryClient set, use QueryClientProvider to set one')
+  }
+  return queryClient
+}
 
 export interface QueryClientProviderProps {
   client: QueryClient