web/admin: remove unused imports

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

web/src/elements/forms/ModelForm.ts

@@ -17,7 +17,11 @@ export abstract class ModelForm<T, PKT extends string | number> extends Form<T>
     private _instancePk?: PKT;
 
     @property({ attribute: false })
-    instance?: T;
+    instance?: T = this.defaultInstance;
+
+    get defaultInstance(): T | undefined {
+        return undefined;
+    }
 
     constructor() {
         super();