Error when search for an item on table

When I insert data in the input and click on the button to make a search give me an error ' Cannot read property 'toString' of null', i insert text or numbers and the error is the same. Someone have a ideia so i can solve that problem? Bellow i attach my code and the error.

Thanks for your time,

Html:

<div class="btn-group">
<input type="text" [(ngModel)]="searchText" placeholder="search" class="form-control" id="search" mdbInput>
</div>
<div class="btn-group">
<button class="btn btn-primary" (click)="searchItems()" ><i class="mdi mdi-15px mdi-magnify"></i></button>
</div>

TypeScript:

searchItems() {
    const prev = this.tableService.getDataSource();

    if (!this.searchText) {
      this.tableService.setDataSource(this.previous);
      this.elements = this.tableService.getDataSource();
    }

    if (this.searchText) {
      this.elements = this.tableService.searchLocalDataBy(this.searchText);
      this.tableService.setDataSource(prev);
    }

  }

Error:

ERROR TypeError: Cannot read property 'toString' of null
    at angular-bootstrap-md.js:8414
    at Array.some (<anonymous>)
    at angular-bootstrap-md.js:8409
    at Array.filter (<anonymous>)
    at MdbTableService.push../node_modules/angular-bootstrap-md/fesm5/angular-bootstrap-md.js.MdbTableService.filterLocalDataBy (angular-bootstrap-md.js:8404)
    at MdbTableService.push../node_modules/angular-bootstrap-md/fesm5/angular-bootstrap-md.js.MdbTableService.searchLocalDataBy (angular-bootstrap-md.js:8431)
    at SimCardsComponent.push../src/app/shared/environment-dashboard/components/simcards/simcards.component.ts.SimCardsComponent.searchItems (simcards.component.ts:140)
    at Object.handleEvent (simcards.component.html:26)
    at handleEvent (core.js:19676)
    at callWithDebugContext (core.js:20770)