transform(werte: any[], kriterium: string, gruppe): any[] {
    if (!werte) {
      return [];
    }
    if (!gruppe || gruppe.length === 0) {
      return werte;
    }
    return werte.filter(it =>
      it[kriterium] === gruppe);
  }

<mat-drawer-container class="example-container">
  <mat-drawer mode="side" opened class="side-nav">
    <!-- <mat-card class="mat-card">Kategorien:</mat-card> -->
    <div *ngFor="let skillGruppe of skillGruppen | unique:'skillGruppe'">
      <button mat-button class="filter">
        <div (click)="filtereSkills(filter.textContent)" #filter>
          {{ skillGruppe.skillGruppe }}
        </div>
      </button>
    </div>
  </mat-drawer>
  <mat-drawer-content>
    <div *ngIf="SKILLS?.length > 0; else noItem">
      <div *ngFor="let skill of SKILLS | filter:'skillGruppe':filterWert">
        <div class="skill">
          <mat-accordion>
            <mat-expansion-panel>
              <mat-expansion-panel-header>
                <mat-panel-title>{{ skill.skillname }} </mat-panel-title>
                <mat-progress-bar
                  class="progress-bar"
                  [value]="skill.skillwert"
                  [color]="'accent'"
                  [mode]="'buffer'"
                ></mat-progress-bar>
                <mat-panel-description> </mat-panel-description>
              </mat-expansion-panel-header>
              <div>{{ skill.skillBeschreibung }}</div>
            </mat-expansion-panel>
          </mat-accordion>
        </div>
      </div>
    </div>
    <ng-template #noItem>
      <app-data-loader></app-data-loader>
    </ng-template>
  </mat-drawer-content>
</mat-drawer-container>