Index: trunk/src/org/openstreetmap/josm/gui/dialogs/ToggleDialog.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/dialogs/ToggleDialog.java	(revision 14902)
+++ trunk/src/org/openstreetmap/josm/gui/dialogs/ToggleDialog.java	(revision 14903)
@@ -888,5 +888,5 @@
 
     protected boolean dockWhenClosingDetachedDlg() {
-        return true;
+        return dialogsPanel != null && titleBar != null;
     }
 
Index: trunk/src/org/openstreetmap/josm/gui/layer/geoimage/ImageViewerDialog.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/layer/geoimage/ImageViewerDialog.java	(revision 14902)
+++ trunk/src/org/openstreetmap/josm/gui/layer/geoimage/ImageViewerDialog.java	(revision 14903)
@@ -507,5 +507,5 @@
         if (collapseButtonClicked) {
             collapseButtonClicked = false;
-            return true;
+            return super.dockWhenClosingDetachedDlg();
         }
         return false;