#vulnerable code 
                              public void generateMergedFile(String filename) {
    FileSystem fs = Injector.INSTANCE.fileSystem();

    File origFile = originalCodebase.getFile(filename);
    boolean origExists = fs.exists(origFile);

    File destFile = destinationCodebase.getFile(filename);
    boolean destExists = fs.exists(destFile);

    File modFile = modifiedCodebase.getFile(filename);
    boolean modExists = fs.exists(modFile);

    if (!destExists && !modExists) {
      // This should never be thrown since generateMergedFile(...) is only called on filesToMerge
      // from merge() which is the union of the files in the destination and modified codebases.
      throw new MoeProblem(
          "%s doesn't exist in either %s nor %s. This should not be possible.",
          filename,
          destinationCodebase,
          modifiedCodebase);

    } else if (origExists && modExists && !destExists) {
      if (areDifferent(filename, origFile, modFile)) {
        // Proceed and merge in /dev/null, which should produce a merge conflict (incoming edit on
        // delete).
        destFile = new File("/dev/null");
      } else {
        // Defer to deletion in destination codebase.
        return;
      }

    } else if (origExists && !modExists && destExists) {
      // Blindly follow deletion of the original file by not copying it into the merged codebase.
      return;

    } else if (!origExists && !(modExists && destExists)) {
      // File exists only in modified or destination codebase, so just copy it over.
      File existingFile = (modExists ? modFile : destFile);
      copyToMergedCodebase(filename, existingFile);
      return;

    } else if (!origExists && modExists && destExists) {
      // Merge both new files (conflict expected).
      origFile = new File("/dev/null");
    }

    File mergedFile = copyToMergedCodebase(filename, destFile);

    try {
      // Merges the changes that lead from origFile to modFile into mergedFile (which is a copy
      // of destFile). After, mergedFile will have the combined changes of modFile and destFile.
      Injector.INSTANCE
          .cmd()
          .runCommand(
              "merge",
              ImmutableList.of(
                  mergedFile.getAbsolutePath(),
                  origFile.getAbsolutePath(),
                  modFile.getAbsolutePath()),
              this.mergedCodebase.getPath().getAbsolutePath());
      // Return status was 0 and the merge was successful. Note it.
      mergedFiles.add(mergedFile.getAbsolutePath());
    } catch (CommandException e) {
      // If merge fails with exit status 1, then a conflict occurred. Make a note of the filepath.
      if (e.returnStatus == 1) {
        failedToMergeFiles.add(mergedFile.getAbsolutePath());
      } else {
        throw new MoeProblem(
            "Merge returned with unexpected status %d when trying to run \"merge -p %s %s %s\"",
            e.returnStatus,
            destFile.getAbsolutePath(),
            origFile.getAbsolutePath(),
            modFile.getAbsolutePath());
      }
    }
  } 
                            #location 5 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                              public void generateMergedFile(String filename) {
    File origFile = originalCodebase.getFile(filename);
    boolean origExists = filesystem.exists(origFile);

    File destFile = destinationCodebase.getFile(filename);
    boolean destExists = filesystem.exists(destFile);

    File modFile = modifiedCodebase.getFile(filename);
    boolean modExists = filesystem.exists(modFile);

    if (!destExists && !modExists) {
      // This should never be thrown since generateMergedFile(...) is only called on filesToMerge
      // from merge() which is the union of the files in the destination and modified codebases.
      throw new MoeProblem(
          "%s doesn't exist in either %s nor %s. This should not be possible.",
          filename,
          destinationCodebase,
          modifiedCodebase);

    } else if (origExists && modExists && !destExists) {
      if (areDifferent(filename, origFile, modFile)) {
        // Proceed and merge in /dev/null, which should produce a merge conflict (incoming edit on
        // delete).
        destFile = new File("/dev/null");
      } else {
        // Defer to deletion in destination codebase.
        return;
      }

    } else if (origExists && !modExists && destExists) {
      // Blindly follow deletion of the original file by not copying it into the merged codebase.
      return;

    } else if (!origExists && !(modExists && destExists)) {
      // File exists only in modified or destination codebase, so just copy it over.
      File existingFile = (modExists ? modFile : destFile);
      copyToMergedCodebase(filename, existingFile);
      return;

    } else if (!origExists && modExists && destExists) {
      // Merge both new files (conflict expected).
      origFile = new File("/dev/null");
    }

    File mergedFile = copyToMergedCodebase(filename, destFile);

    try {
      // Merges the changes that lead from origFile to modFile into mergedFile (which is a copy
      // of destFile). After, mergedFile will have the combined changes of modFile and destFile.
      cmd.runCommand(
          "merge",
          ImmutableList.of(
              mergedFile.getAbsolutePath(), origFile.getAbsolutePath(), modFile.getAbsolutePath()),
          this.mergedCodebase.getPath().getAbsolutePath());
      // Return status was 0 and the merge was successful. Note it.
      mergedFiles.add(mergedFile.getAbsolutePath());
    } catch (CommandException e) {
      // If merge fails with exit status 1, then a conflict occurred. Make a note of the filepath.
      if (e.returnStatus == 1) {
        failedToMergeFiles.add(mergedFile.getAbsolutePath());
      } else {
        throw new MoeProblem(
            "Merge returned with unexpected status %d when trying to run \"merge -p %s %s %s\"",
            e.returnStatus,
            destFile.getAbsolutePath(),
            origFile.getAbsolutePath(),
            modFile.getAbsolutePath());
      }
    }
  }