ngOnDestroy Uncaught (in promise): TypeError: Cannot read property 'unsubscribe' of undefined

I'm getting an error when unsubscribing during the ngOnDestroy method in my shopping-edit.component.ts when i click the link to go to my recipes page. here is the image

error on link click

here is a link to my git https://github.com/CHBaker/First-Angular-App

and my code snippet:

import {
    Component,
    OnInit,
    OnDestroy,
    ViewChild
} from '@angular/core';
import { NgForm } from '@angular/forms';
import { Subscription } from 'rxjs/Subscription';


import { Ingredient } from '../../shared/ingredient.model';
import { ShoppingListService } from '../shopping-list.service';

@Component({
    selector: 'app-shopping-edit',
    templateUrl: './shopping-edit.component.html',
    styleUrls: ['./shopping-edit.component.css']
})
export class ShoppingEditComponent implements OnInit, OnDestroy {
    @ViewChild('f') slForm: NgForm;
    subscription: Subscription;
    editMode = false;
    editedItemIndex: number;
    editedItem: Ingredient;

    constructor(private slService: ShoppingListService) { }

    ngOnInit() {
        this.slService.startedEditing
            .subscribe(
                (index: number) => {
                    this.editedItemIndex = index;
                    this.editMode = true;
                    this.editedItem = this.slService.getIngredient(index);
                    this.slForm.setValue({
                        name: this.editedItem.name,
                        amount: this.editedItem.amount
                    })
                }
            );
    }

    onSubmit(form: NgForm) {
        const value = form.value;
        const newIngredient = new Ingredient(value.name, value.amount);
        if (this.editMode) {
            this.slService.updateIngredient(this.editedItemIndex, newIngredient);
        } else {
            this.slService.addIngredient(newIngredient);
        }
        this.editMode = false;
        form.reset();
    }

    onClear() {
        this.slForm.reset();
        this.editMode = false;
    }

    onDelete() {
        this.slService.deleteIngredient(this.editedItemIndex);
        this.onClear();
    }

    ngOnDestroy() {
        this.subscription.unsubscribe();
    }
}