Fix for cryptomator#2989 - improved code readability and exception

- better use of the Optional class
- throws IllegalArgumentException is more appropriate when a required object is not present

src/main/java/org/cryptomator/ui/error/ErrorController.java

@@ -157,9 +157,7 @@ private void loadHttpResponse(HttpResponse<InputStream> response) {
 								.thenComparing(this::compareIsAnswered)//
 								.thenComparing(this::compareUpvoteCount));
 
-				if (value.isPresent()) {
-					matchingErrorDiscussion.set(value.get());
-				}
+				value.ifPresent(matchingErrorDiscussion::set);
 			}
 		} catch (IOException e) {
 			LOG.error("Failed to load or parse JSON from " + response.uri(), e);

src/main/java/org/cryptomator/ui/fxapp/UpdateCheckerModule.java

@@ -81,16 +81,8 @@ static ScheduledService<String> provideCheckForUpdatesService(ExecutorService ex
 		ScheduledService<String> service = new ScheduledService<>() {
 			@Override
 			protected Task<String> createTask() {
-				if (httpClient.isPresent()) {
-					return new UpdateCheckerTask(httpClient.get(), checkForUpdatesRequest);
-				} else {
-					return new Task<>() {
-						@Override
-						protected String call() {
-							throw new NullPointerException("No HttpClient present.");
-						}
-					};
-				}
+				return httpClient.map(client -> new UpdateCheckerTask(client, checkForUpdatesRequest))
+						.orElseThrow(() -> new IllegalArgumentException("No HttpClient present."));
 			}
 		};
 		service.setOnFailed(event -> LOG.error("Failed to execute update service", service.getException()));