Index: /trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java	(revision 11588)
+++ /trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java	(revision 11589)
@@ -752,5 +752,5 @@
                     Main.fileWatcher.registerValidatorRule(source);
                 }
-            } catch (IOException ex) {
+            } catch (IOException | IllegalStateException ex) {
                 Main.warn(tr("Failed to add {0} to tag checker", i));
                 Main.warn(ex, false);
Index: /trunk/src/org/openstreetmap/josm/gui/mappaint/MapPaintStyles.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/gui/mappaint/MapPaintStyles.java	(revision 11588)
+++ /trunk/src/org/openstreetmap/josm/gui/mappaint/MapPaintStyles.java	(revision 11589)
@@ -274,5 +274,5 @@
             try {
                 Main.fileWatcher.registerStyleSource(source);
-            } catch (IOException e) {
+            } catch (IOException | IllegalStateException e) {
                 Main.error(e);
             }
Index: /trunk/src/org/openstreetmap/josm/io/FileWatcher.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/io/FileWatcher.java	(revision 11588)
+++ /trunk/src/org/openstreetmap/josm/io/FileWatcher.java	(revision 11589)
@@ -52,5 +52,5 @@
      */
     public final void start() {
-        if (!thread.isAlive()) {
+        if (thread != null && !thread.isAlive()) {
             thread.start();
         }