hotfix for watch not defined

lib/core/engine.js

@@ -146,7 +146,9 @@ class Engine {
         self.currentAbi = abi;
         self.contractsJSON = contractsJSON;
         pipeline.build(abi, contractsJSON, null, function() {
+          if (self.watch) {
             self.watch.restart(); // Necessary because changing a file while it is writing can stop it from being watched
+          }
           self.events.emit('outputDone');
         });
       });