fix mem-fs change event on refresh

Author: mshima

src/environment-base.ts

@@ -875,7 +875,7 @@ export default class EnvironmentBase extends EventEmitter implements BaseEnviron
           // Conflicter can change files add listener before commit task.
           const changedFileHandler = (filePath: string) => {
             const file = this.sharedFs.get(filePath);
-            if (isFilePending(file)) {
+            if (file && isFilePending(file)) {
               queueCommit();
               this.sharedFs.removeListener('change', changedFileHandler);
             }