Index: trunk/src/org/openstreetmap/josm/Main.java
===================================================================
--- trunk/src/org/openstreetmap/josm/Main.java	(revision 10399)
+++ trunk/src/org/openstreetmap/josm/Main.java	(revision 10400)
@@ -102,4 +102,5 @@
 import org.openstreetmap.josm.gui.progress.ProgressMonitorExecutor;
 import org.openstreetmap.josm.gui.tagging.presets.TaggingPresets;
+import org.openstreetmap.josm.gui.util.GuiHelper;
 import org.openstreetmap.josm.gui.util.RedirectInputMap;
 import org.openstreetmap.josm.gui.widgets.JMultilineLabel;
@@ -809,5 +810,5 @@
      * the viewport isn't changed
      */
-    public final synchronized void addLayer(final Layer layer, ProjectionBounds bounds) {
+    public final void addLayer(Layer layer, ProjectionBounds bounds) {
         addLayer(layer, bounds == null ? null : new ViewportData(bounds));
     }
@@ -821,5 +822,5 @@
      * @param viewport the viewport to zoom to; can be null, then the viewport isn't changed
      */
-    public final synchronized void addLayer(final Layer layer, ViewportData viewport) {
+    public final void addLayer(Layer layer, ViewportData viewport) {
         getLayerManager().addLayer(layer);
         if (viewport != null) {
@@ -828,5 +829,11 @@
     }
 
+    /**
+     * Creates the map frame. Call only in EDT Thread.
+     * @param firstLayer The first layer that was added.
+     * @param viewportData The initial viewport. Can be <code>null</code> to be automatically computed.
+     */
     public synchronized void createMapFrame(Layer firstLayer, ViewportData viewportData) {
+        GuiHelper.assertCallFromEdt();
         MapFrame mapFrame = new MapFrame(contentPanePrivate, viewportData);
         setMapFrame(mapFrame);