Index: trunk/src/org/openstreetmap/josm/gui/dialogs/relation/actions/PasteMembersAction.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/relation/actions/PasteMembersAction.java	(revision 11677)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/relation/actions/PasteMembersAction.java	(revision 11678)
@@ -5,4 +5,5 @@
 import java.awt.datatransfer.FlavorListener;
 import java.awt.event.ActionEvent;
+import java.util.Optional;
 
 import javax.swing.TransferHandler.TransferSupport;
@@ -42,5 +43,6 @@
 
     private TransferSupport getSupport() {
-        return new TransferSupport(memberTable, ClipboardUtils.getClipboardContent());
+        return new TransferSupport(memberTable, Optional.ofNullable(ClipboardUtils.getClipboardContent())
+                .orElseThrow(() -> new IllegalStateException("Failed to retrieve clipboard content")));
     }