Made MainScreen class public.

src/Main.java

 import javafx.application.Application;
 import javafx.stage.Stage;
 import model.OSInterface;
-import ui.mainScreen;
+import ui.MainScreen;
 
 public class Main extends Application {
 
     @Override
     public void start(Stage primaryStage) throws Exception{
         // Initialize at main UI screen
-        new mainScreen();
+        new MainScreen();
 
 
     }

src/ui/MainScreen.java

@@ -10,7 +10,7 @@ import javafx.stage.Stage;
  * mainScreen is the main UI page for the program. All user functions start here.
  */
 
-class MainScreen extends Pane {
+public class MainScreen extends Pane {
         Stage primaryStage = new Stage();
         Scene mainScreenScene = new Scene(this, 800, 800);