fix: Remove getUser in callback handler (#461)

Makes state available after `signinCallback` to onSignIn event

fixes #447 

Co-authored-by: Edward Sparkes <sparkes.e@cambridgeassessment.org.uk>

src/AuthContext.tsx

@@ -85,8 +85,7 @@ export const AuthProvider: FC<AuthProviderProps> = ({
        * Check if the user is returning back from OIDC.
        */
       if (hasCodeInUrl(location)) {
-        await userManager.signinCallback();
-        const user = await userManager.getUser();
+        const user = await userManager.signinCallback();
         setUserData(user);
         onSignIn && onSignIn(user);
         return;