Index: trunk/src/org/openstreetmap/josm/io/FileWatcher.java
===================================================================
--- trunk/src/org/openstreetmap/josm/io/FileWatcher.java	(revision 12813)
+++ trunk/src/org/openstreetmap/josm/io/FileWatcher.java	(revision 12814)
@@ -5,4 +5,5 @@
 import java.io.IOException;
 import java.nio.file.FileSystems;
+import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.StandardWatchEventKinds;
@@ -136,4 +137,14 @@
                 Path fullPath = ((Path) key.watchable()).resolve(filename);
 
+                try {
+                    // Some filesystems fire two events when a file is modified. Skip first event (file is empty)
+                    if (Files.size(fullPath) == 0) {
+                        continue;
+                    }
+                } catch (IOException ex) {
+                    Logging.error(ex);
+                    continue;
+                }
+
                 synchronized (this) {
                     StyleSource style = styleMap.get(fullPath);