Styling: replaced all occurrences of

`return new UnknownStyleException( key )`
with
  `throw new UnknownStyleException( key )`

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

@@ -370,7 +370,7 @@ public class FlatButtonUI
 	protected Object applyStyleProperty( AbstractButton b, String key, Object value ) {
 		if( key.startsWith( "help." ) ) {
 			if( !(helpButtonIcon instanceof FlatHelpButtonIcon) )
-				return new UnknownStyleException( key );
+				throw new UnknownStyleException( key );
 
 			if( helpButtonIconShared ) {
 				helpButtonIcon = FlatStylingSupport.cloneIcon( helpButtonIcon );

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

@@ -205,7 +205,7 @@ public class FlatRadioButtonUI
 		if( key.startsWith( "icon." ) ) {
 			Icon icon = getRealIcon( b );
 			if( !(icon instanceof FlatCheckBoxIcon) )
-				return new UnknownStyleException( key );
+				throw new UnknownStyleException( key );
 
 			if( icon == this.icon && iconShared ) {
 				this.icon = icon = FlatStylingSupport.cloneIcon( icon );

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

@@ -671,7 +671,7 @@ public class FlatTabbedPaneUI
 		// close icon
 		if( key.startsWith( "close" ) ) {
 			if( !(closeIcon instanceof FlatTabbedPaneCloseIcon) )
-				return new UnknownStyleException( key );
+				throw new UnknownStyleException( key );
 
 			if( closeIconShared ) {
 				closeIcon = FlatStylingSupport.cloneIcon( closeIcon );

flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyleableValue.java

@@ -77,6 +77,7 @@ import com.formdev.flatlaf.icons.FlatRadioButtonMenuItemIcon;
 import com.formdev.flatlaf.icons.FlatSearchIcon;
 import com.formdev.flatlaf.icons.FlatSearchWithHistoryIcon;
 import com.formdev.flatlaf.ui.FlatStylingSupport.StyleableUI;
+import com.formdev.flatlaf.ui.FlatStylingSupport.UnknownStyleException;
 import com.formdev.flatlaf.ui.TestFlatStyling.CustomCheckBoxIcon;
 import com.formdev.flatlaf.ui.TestFlatStyling.CustomIcon;
 import com.formdev.flatlaf.ui.TestFlatStyling.CustomRadioButtonIcon;
@@ -574,6 +575,12 @@ public class TestFlatStyleableValue
 		//---- icon ----
 
 		if( b.getIcon() instanceof CustomIcon ) {
+			try {
+				ui.applyStyle( b, "icon.focusWidth: 1.23" );
+				assertTrue( false );
+			} catch( UnknownStyleException ex ) {
+				assertEquals( new UnknownStyleException( "icon.focusWidth" ).getMessage(), ex.getMessage() );
+			}
 			assertEquals( null, ui.getStyleableValue( b, "icon.focusWidth" ) );
 			return;
 		}

flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyling.java

@@ -33,6 +33,7 @@ import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 import com.formdev.flatlaf.FlatLaf;
 import com.formdev.flatlaf.icons.*;
+import com.formdev.flatlaf.ui.FlatStylingSupport.UnknownStyleException;
 import com.formdev.flatlaf.util.ColorFunctions;
 
 /**
@@ -697,8 +698,15 @@ public class TestFlatStyling
 
 		//---- icon ----
 
-		if( b.getIcon() instanceof CustomIcon )
+		if( b.getIcon() instanceof CustomIcon ) {
+			try {
+				ui.applyStyle( b, "icon.focusWidth: 1.5" );
+				assertTrue( false );
+			} catch( UnknownStyleException ex ) {
+				assertEquals( new UnknownStyleException( "icon.focusWidth" ).getMessage(), ex.getMessage() );
+			}
 			return;
+		}
 
 		ui.applyStyle( b, "icon.focusWidth: 1.5" );
 		ui.applyStyle( b, "icon.focusColor: #fff" );