Call to a member function swap() on null - Laravel cashier subscription

I am creating a subscription using laravel cashier and stripe. I am not using the user model, but using employer model. I have added the billable trait, set CASHIER_MODEL=App\Employer in my .env. Done all that is required but I get below error when making payments.

Call to a member function swap() on null

My codes

try {

	// get plan
	$plan = Plan::where('slug', $request->slug)->first();
			
	$getEmployer = auth('employer')->user(); 

	$stripeCustomer = $getEmployer->createOrGetStripeCustomer();

	$updateStripeCustomer =  $getEmployer->updateStripeCustomer([
			'name' => $getEmployer->fname ." " .$getEmployer->lname,
			'email' => $getEmployer->email,
			'phone' => $getEmployer->phone,
		]);
			
	// get paymentMethod
	$paymentMethod = $request->paymentMethod;

	if ($getEmployer->hasDefaultPaymentMethod()) {
		$getEmployer->updateDefaultPaymentMethod($paymentMethod);
	}else{
		$getEmployer->addPaymentMethod($paymentMethod);
	}

	// check if user is already subscribed, then swap
	if ($getEmployer->subscribed('main')) {
		// charge if new subscriber
		$charge =  $getEmployer->newSubscription('main', $plan->stripe_plan)->create($paymentMethod, [
					'email' => $getEmployer->email,
				]);

		// save new subscription status
		$getEmployer->plan_id = $plan->id;
		$getEmployer->save();

      		 // Mail
		event(new SubscriptionMail($getEmployer));

		\Session::flash('success', 'You have upgraded to the '.$plan->plan_type.' plan');
		return redirect()->route('employer.dashboard');
				
	}else{

		$getEmployer->subscription('main')->swap($plan->stripe_plan);
		event(new SubscriptionMail($getEmployer));

		\Session::flash('success', 'You have changed to the '.$plan->plan_type.' plan');
		return redirect()->route('employer.dashboard');
		}
} catch (\Exception $e) {
	\Session::flash('error',$e->getMessage());
return back();
}