static Object lazyUIManagerGet( String uiKey ) {
		boolean optional = false;
		if( uiKey.startsWith( OPTIONAL_PREFIX ) ) {
			uiKey = uiKey.substring( OPTIONAL_PREFIX.length() );
			optional = true;
		}

		Object value = UIManager.get( uiKey );
		if( value == null && !optional )
			LoggingFacade.INSTANCE.logSevere( "FlatLaf: '" + uiKey + "' not found in UI defaults.", null );
		return value;
	}
For use in LazyValue to get value for given key from UIManager and report error
if not found. If key is prefixed by '?', then no error is reported.
flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java