Add check to watcher, to avoid null-error if unknown path is removed.

pkg/watcher/lib/src/path_set.dart

Index: pkg/watcher/lib/src/path_set.dart
diff --git a/pkg/watcher/lib/src/path_set.dart b/pkg/watcher/lib/src/path_set.dart
index 01d4208f4f79ab86661ec87348afdfcfc41ed9b8..e9f7d32d25c294ea697cb702752e63ac21112584 100644
--- a/pkg/watcher/lib/src/path_set.dart
+++ b/pkg/watcher/lib/src/path_set.dart
@@ -71,7 +71,7 @@ class PathSet {
         // the next level.
         var part = parts.removeFirst();
         var entry = dir[part];
-        if (entry.isEmpty) return new Set();
+        if (entry == null || entry.isEmpty) return new Set();
 
         partialPath = p.join(partialPath, part);
         var paths = recurse(entry, partialPath);