Index: /trunk/src/org/openstreetmap/josm/io/session/SessionReader.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/io/session/SessionReader.java	(revision 11042)
+++ /trunk/src/org/openstreetmap/josm/io/session/SessionReader.java	(revision 11043)
@@ -292,4 +292,10 @@
             return layerDependencies;
         }
+
+        @Override
+        public String toString() {
+            return "ImportSupport [layerName=" + layerName + ", layerIndex=" + layerIndex + ", layerDependencies="
+                    + layerDependencies + ", inZipPath=" + inZipPath + ']';
+        }
     }
 
@@ -489,5 +495,8 @@
                 try {
                     layer = imp.load(e, support, progressMonitor.createSubTaskMonitor(1, false));
-                } catch (IllegalDataException | IOException ex) {
+                    if (layer == null) {
+                        throw new IllegalStateException("Importer " + imp + " returned null for " + support);
+                    }
+                } catch (IllegalDataException | IllegalStateException | IOException ex) {
                     exception = ex;
                 }
@@ -511,5 +520,4 @@
                 }
 
-                if (layer == null) throw new RuntimeException();
                 layersMap.put(idx, layer);
             }