fix build failure

src/bin/mdbook.rs

@@ -358,7 +358,10 @@ fn trigger_on_change<F>(book: &mut MDBook, closure: F) -> ()
     };
 
     // Add the theme directory to the watcher
-    watcher.watch(book.get_theme_path(), Recursive).unwrap_or_default();
+    if let Some(t) = book.get_theme_path() {
+        watcher.watch(t, Recursive).unwrap_or_default();
+    }
+
 
     // Add the book.{json,toml} file to the watcher if it exists, because it's not
     // located in the source directory