Improve adminClient handling in Controller

* Closing previous adminClient if present and clear TopicList, before creating a new one
* Display Exceptions thrown during AdminClient creation
* Clear cluster selection if AdminClient creation failed

src/main/java/at/esque/kafka/Controller.java

@@ -289,8 +289,17 @@ public void setup(Stage controlledStage) {
         });
 
         clusterComboBox.getSelectionModel().selectedItemProperty().addListener((observable, oldValue, newValue) -> {
-            adminClient = new KafkaesqueAdminClient(newValue.getBootStrapServers(), configHandler.getSslProperties(selectedCluster()), configHandler.getSaslProperties(selectedCluster()));
-            refreshTopicList();
+            try {
+                closeAdminClientIfPresentAndClearTopicList();
+                if (newValue == null) {
+                    return;
+                }
+                adminClient = new KafkaesqueAdminClient(newValue.getBootStrapServers(), configHandler.getSslProperties(selectedCluster()), configHandler.getSaslProperties(selectedCluster()));
+                refreshTopicList();
+            } catch (Exception e) {
+                ErrorAlert.show(e, controlledStage);
+                clusterComboBox.getSelectionModel().clearSelection();
+            }
         });
 
         partitionCombobox.getItems().add(-1);
@@ -345,6 +354,14 @@ public void setup(Stage controlledStage) {
         });
     }
 
+    private void closeAdminClientIfPresentAndClearTopicList() {
+        if (adminClient != null) {
+            adminClient.close();
+            adminClient = null;
+            topicListView.getBaseList().clear();
+        }
+    }
+
     private void showTextInStageTitle(String text) {
         if (Strings.isNullOrEmpty(text)) {
             controlledStage.setTitle("Kafkaesque");