Fixed NPE when trying to access drop target in headless mode.

Fixes #119

core/src/main/java/com/github/weisj/darklaf/ui/tabbedpane/DarkHandler.java

@@ -29,6 +29,7 @@ import javax.swing.*;
 import javax.swing.event.ChangeEvent;
 import javax.swing.plaf.UIResource;
 import java.awt.*;
+import java.awt.dnd.DropTarget;
 import java.awt.event.ContainerEvent;
 import java.awt.event.MouseEvent;
 import java.beans.PropertyChangeEvent;
@@ -108,7 +109,8 @@ public class DarkHandler extends TabbedPaneHandler {
             ui.tabPane.doLayout();
         } else if (DarkTabbedPaneUI.KEY_DND.equals(key)) {
             ui.dndEnabled = Boolean.TRUE.equals(ui.tabPane.getClientProperty(DarkTabbedPaneUI.KEY_DND));
-            ui.tabPane.getDropTarget().setActive(ui.dndEnabled);
+            DropTarget dropTarget = ui.tabPane.getDropTarget();
+            if (dropTarget != null) dropTarget.setActive(ui.dndEnabled);
         } else if (PropertyKey.COMPONENT_ORIENTATION.equals(key)) {
             ui.tabPane.doLayout();
             ui.tabPane.repaint();

core/src/main/java/com/github/weisj/darklaf/ui/tabbedpane/DarkTabbedPaneUI.java

@@ -32,6 +32,7 @@ import javax.swing.*;
 import javax.swing.plaf.ComponentUI;
 import javax.swing.plaf.UIResource;
 import java.awt.*;
+import java.awt.dnd.DropTarget;
 import java.awt.event.AWTEventListener;
 import java.awt.event.FocusEvent;
 import java.awt.event.FocusListener;
@@ -147,8 +148,11 @@ public class DarkTabbedPaneUI extends DarkTabbedPaneUIBridge {
     protected void installDragSupport() {
         tabPane.setTransferHandler(TRANSFER_HANDLER);
         try {
-            tabPane.getDropTarget().addDropTargetListener(TRANSFER_HANDLER);
-            tabPane.getDropTarget().setActive(dndEnabled);
+            DropTarget target = tabPane.getDropTarget();
+            if (target != null) {
+                target.addDropTargetListener(TRANSFER_HANDLER);
+                target.setActive(dndEnabled);
+            }
         } catch (TooManyListenersException e) {
             e.printStackTrace();
         }

core/src/main/java/com/github/weisj/darklaf/ui/tabframe/DarkTabFrameUI.java

@@ -35,6 +35,7 @@ import org.pbjar.jxlayer.plaf.ext.transform.TransformUtils;
 import javax.swing.*;
 import javax.swing.plaf.ComponentUI;
 import java.awt.*;
+import java.awt.dnd.DropTarget;
 import java.awt.event.AWTEventListener;
 import java.awt.event.MouseEvent;
 import java.util.List;
@@ -144,8 +145,11 @@ public class DarkTabFrameUI extends TabFrameUI implements AWTEventListener {
     protected void installDnD() {
         tabFrame.setTransferHandler(TRANSFER_HANDLER);
         try {
-            tabFrame.getDropTarget().addDropTargetListener(TRANSFER_HANDLER);
-            tabFrame.getDropTarget().setActive(tabFrame.isDndEnabled());
+            DropTarget dropTarget = tabFrame.getDropTarget();
+            if (dropTarget != null) {
+                dropTarget.addDropTargetListener(TRANSFER_HANDLER);
+                dropTarget.setActive(tabFrame.isDndEnabled());
+            }
         } catch (TooManyListenersException e) {
             e.printStackTrace();
         }