Fix NPE in ScreenShotPlugin

Fix NPE that happens when titlebar is not present (not using the custom
window decorations) and plugin is trying to remove the screenshot
button from titlebar.

Signed-off-by: Tomas Slusny <slusnucky@gmail.com>

runelite-client/src/main/java/net/runelite/client/plugins/screenshot/ScreenshotPlugin.java

@@ -169,10 +169,13 @@ public class ScreenshotPlugin extends Plugin
 		SwingUtilities.invokeLater(() ->
 		{
 			JComponent titleBar = SubstanceCoreUtilities.getTitlePaneComponent(clientUi);
-			titleBar.remove(titleBarButton);
 
-			clientUi.revalidate();
-			clientUi.repaint();
+			if (titleBar != null)
+			{
+				titleBar.remove(titleBarButton);
+				clientUi.revalidate();
+				clientUi.repaint();
+			}
 		});
 	}