CheckBox and RadioButton: fixed NPE when button has children (similar to PR #68)

CHANGELOG.md

@@ -6,6 +6,7 @@ FlatLaf Change Log
 - TextField, FormattedTextField and PasswordField: Support round borders (see UI
   default value `TextComponent.arc`). (issue #65)
 - IntelliJ Themes: Added Gradianto themes to demo.
+- Button, CheckBox and RadioButton: Fixed NPE when button has children. (PR #68)
 
 
 ## 0.27

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatRadioButtonUI.java

@@ -101,6 +101,8 @@ public class FlatRadioButtonUI
 	@Override
 	public Dimension getPreferredSize( JComponent c ) {
 		Dimension size = super.getPreferredSize( c );
+		if( size == null )
+			return null;
 
 		// small insets fix
 		int focusWidth = getIconFocusWidth( c );