Popup: made Popup.show(), hide() and component listener more robust when used in unusual ways (issue #106)

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

@@ -153,6 +153,7 @@ public class FlatPopupFactory
 
 		@Override
 		public void show() {
+			if( delegate != null )
 				delegate.show();
 		}
 
@@ -185,6 +186,7 @@ public class FlatPopupFactory
 		private boolean oldOpaque;
 
 		// medium weight
+		private boolean mediumWeightShown;
 		private Panel mediumWeightPanel;
 		private JPanel dropShadowPanel;
 		private ComponentListener mediumPanelListener;
@@ -311,6 +313,11 @@ public class FlatPopupFactory
 		}
 
 		private void showMediumWeightDropShadow() {
+			if( mediumWeightShown )
+				return;
+
+			mediumWeightShown = true;
+
 			Window window = SwingUtilities.windowForComponent( owner );
 			if( window == null )
 				return;
@@ -326,23 +333,28 @@ public class FlatPopupFactory
 			mediumPanelListener = new ComponentListener() {
 				@Override
 				public void componentShown( ComponentEvent e ) {
+					if( dropShadowPanel != null )
 						dropShadowPanel.setVisible( true );
 				}
 
 				@Override
 				public void componentHidden( ComponentEvent e ) {
+					if( dropShadowPanel != null )
 						dropShadowPanel.setVisible( false );
 				}
 
 				@Override
 				public void componentMoved( ComponentEvent e ) {
+					if( dropShadowPanel != null && mediumWeightPanel != null ) {
 						Point location = mediumWeightPanel.getLocation();
 						Insets insets = dropShadowPanel.getInsets();
 						dropShadowPanel.setLocation( location.x - insets.left, location.y - insets.top );
 					}
+				}
 
 				@Override
 				public void componentResized( ComponentEvent e ) {
+					if( dropShadowPanel != null )
 						dropShadowPanel.setSize( FlatUIUtils.addInsets( mediumWeightPanel.getSize(), dropShadowPanel.getInsets() ) );
 				}
 			};