web/admin: don't use form.reset() for ModelForms, reset instance

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

web/src/elements/forms/ModelForm.ts

@@ -39,7 +39,7 @@ export abstract class ModelForm<T, PKT extends string | number> extends Form<T>
     }
 
     resetForm(): void {
-        super.resetForm();
+        this.instance = undefined;
         this._initialLoad = false;
     }