webadmin: Fix template import

The import of the template failed because one
of the provided listeners on cluster changes
was null. The listener was used to load OS into
a list for the VM, however, it is not needed
for the templates. This patch adds a check and
does not add the listener if it is null (as is
the case for the templates).

Bug-Url: https://bugzilla.redhat.com/2104597

frontend/webadmin/modules/uicommonweb/src/main/java/org/ovirt/engine/ui/uicommonweb/models/vms/ImportVmFromExportDomainModel.java

@@ -82,7 +82,9 @@ protected void doInit() {
         }
         getClusterQuota().setIsAvailable(dataCenter.getQuotaEnforcementType() != QuotaEnforcementTypeEnum.DISABLED);
         getCluster().getSelectedItemChangedEvent().addListener(clusterChangedListener);
-        getCluster().getSelectedItemChangedEvent().addListener(vmImportGeneralModel);
+        if (vmImportGeneralModel != null) {
+            getCluster().getSelectedItemChangedEvent().addListener(vmImportGeneralModel);
+        }
         // get cluster
         getCluster().setItems(null);
         AsyncDataProvider.getInstance().getClusterByServiceList(new AsyncQuery<>(clusters -> {