[24129] Do reset the form on successful saves

If we do not reset the form after sucessful saving, subsequent editing
fields WILL receive the old form, which results in invalid data being
shown.

For example, changing the status of a work package from A to B with a workflow
that defines a new transition from B to C, then status C will not be
shown if only traversable from B.

frontend/app/components/api/api-v3/hal-resources/work-package-resource.service.ts

@@ -281,6 +281,10 @@ export class WorkPackageResource extends HalResource {
               delete this.$pristine[key];
             });
 
+            // Remove the current form, otherwise old form data
+            // might still be used for the next edit field in #getSchema()
+            this.form = null;
+
             // Forcefully refresh activities
             wpCacheService.loadWorkPackageLinks(this, 'activities');