How to collect data using multiple eager loading in laravel 5

I'm just wondering why my code is not working, I mean, after I collect single data using eager loading, after that, if I collect the nested relationship, it returns an error. Please see my code below.

Error

Trying to get property of non-object

My Code

Controller.php

$exam = Exam::with('answers', 'child.answers')->find($id);
$exam = collect($exam->answers);
$exams = $quizzes->merge(collect($exam->child->answers));

return view('school.exam.view', compact('exams'));

view.blade.php

@foreach($exams as $exam)
  <li></li>
@endforeach

What should I do with this?

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