Laravel Trying to access array offset on value of type null

After deleting one adviser. this error shows "Trying to access array offset on value of type null". can you help me fix this. thank you.

Adviser model:

public function students()
    {
        return $this->hasMany(Student::class);
    }

public function getAdviserNameAttribute()
    {
        return $this->first_name.' '.$this->last_name;
    }

Student model:

public function advisers()
    {
        return $this->belongsTo(Adviser::class, 'adviser_id');
    }

student migration:

$table->BigInteger('adviser_id')->unsigned()->nullable();
$table->foreign('adviser_id')->references('id')->on('advisers')->onDelete('set null');

index.blade.view:

<td>
	{{ $student->advisers['advisername'] }}							
</td>	

controller:

  public function index()
    {
        $students = Student::paginate(5);
        $users = User::all();
        $advisers = Adviser::all();
       
        return view ('admin.index', compact('students', 'users', 'advisers'));
    }

//delete adviser
     public function deleteAdviser(Adviser $adviser)
    {
        $adviser->delete();

        return redirect()->back()->with("error","Deleted!");
    }