Laravel 6 onDelete->（'set null'）不起作用

Question

我的CRUD应用程序出现奇怪的错误。 onUpdate->('cascade')有效，但不适用于onDelete->('set null')。如果我在数据库中手动删除它，它会起作用，但不适用于控制器中的删除操作。请在下面查看我的迁移文件。

class CreatePositionsTable extends Migration
{
    public function up()
    {
        Schema::create('positions', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->bigInteger('department_id')->unsigned()->nullable();
            $table->foreign('department_id')->references('id')->on('departments')
                ->onDelete('set null')->onUpdate('cascade');
            $table->bigInteger('section_id')->unsigned()->nullable();
            $table->foreign('section_id')->references('id')->on('sections')
                ->onDelete('set null')->onUpdate('cascade');
            $table->string('position_name', 100);
            $table->string('n_level', 10)->nullable();
            $table->string('job_description')->nullable();
            $table->softDeletes();
            $table->timestamps();
        });
    }

    public function down()
    {
        Schema::dropIfExists('positions');
    }
}

我的模型（如果有影响）

Positions.php

 <?php

namespace App;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\SoftDeletes;
use Iatstuti\Database\Support\CascadeSoftDeletes; //composer require iatstuti/laravel-cascade-soft-deletes

use App\SalaryMatrix;
use App\Employee;

class Position extends Model
{
    protected $fillable = ['department_id', 'section_id', 'position_name', 'n_level', 'job_description'];

    use SoftDeletes, CascadeSoftDeletes;
    protected $softCascade = ['deleted_at'];
    protected $cascadeDeletes = ['salaryMatrix', 'employee'];

    public function department () {
        return $this->belongsTo('App\Department');
    }

    public function section () {
        return $this->belongsTo('App\Section');
    }

    public function salaryMatrix() {
        return $this->hasOne('App\SalaryMatrix', 'position_id');
    }

    public function employee() {
        return $this->hasMany('App\Employee', 'position_id');
    }
}

PositionController.php

 public function destroy($id) {
    $position = Position::findOrFail($id);
    $position->delete();
  }

注意：此项目仅部署在localhost中。