Edit, save and modify GUI data (Clientside) - MC version 1.16.5 forge

I created a feature in my mod to save and open gui's, but I want to dig into the data of the gui and look and what I can change and do there.

Pasting GUI variable in chat gives an error, storing it in a txt file leaves the file empty, and trying to save the data to a new string variable also returns an error. 

Note that the GUI is a "screen" variable type, implemented by net.minecraft.client.gui.screen

Some more information about my code:

// GUI variable
private static Screen savedGUIScreen = null;

// Saving the GUI
if (Minecraft.getInstance().currentScreen != null) {
                savedGUIScreen = screen;
}

// One approach which left the file empty:
try {
                    FileWriter writer2 = new FileWriter("writerTESTING.txt");
                    writer2.write("TEST1234 done please make it work :)\n");
                    writer2.write(savedGUIScreen.getClass().getName());
                } catch (IOException e) {
                    Minecraft.getInstance().player.sendChatMessage("An error occured while trying to save GUI:");
                    e.printStackTrace();
                }
// writerTESTING.txt is still an empty 0 KB file