Wsrtjtyk

        $employee = Employee::where('user_id', Auth::user()->id)->get();



            foreach (Company::all() as $company)

                {

                    if ($company->id == $employee[0]->company_id && $company->employee_active === 1)

                    {

                        $event->menu->add([

                            'text'        => 'Contracten',

                            'url'         => 'dashboard/contracts',

                            'icon'        => 'file-text',

                            'submenu' => [

                                [

                                    'text' => 'Contract opzetten',

                                    'url'  => 'dashboard/contracts/create',

                                    'icon_color' => 'red',

                                ]

                            ]

                        ]);

                    }

                }

When I use this code I'm getting undefined offset: 0, if the database is empty. How can get this write? Should I use an if or something like that

Really you should configure your relationships correctly so that you can do

$companies = Company::all();



foreach($companies as $company){



    foreach($company->employees as $employee){



        if($employee->active){

          ....

        }   



    }



}

But in your case you can change your first line to

 $employee = Employee::where('user_id', Auth::user()->id)->first();

Which will return an Employee object collection rather than an array so you don't need to use an [index] to get the first object.

@Joe solution is correct. You can make an eager loading of companies and employees to avoid N+1 query problem. There is an official example of this case:
https://laravel.com/docs/5.7/eloquent-relationships#eager-loading

You should leverage constrained eager loading to limit your results. This will return only the companies where the user is an employee.

$companies = Company::where('employee_active', 1)->whereHas('employees', function($query) {

   $query->where('user_id', auth()->id());

})->get();

Now you dont need the conditional checks for generating the menu items.