fix check subscription exists before unsubscribing

Author: pturner1989

lib/ngx-drop/file-drop.component.ts

@@ -151,7 +151,9 @@ export class FileComponent {
   }
 
   ngOnDestroy() {
-    this.subscription.unsubscribe();
+    if(this.subscription) {
+      this.subscription.unsubscribe();
+    }
   }
 }