Notice: Trying to get property of non-object in gmap_regenerate_markers() (line 555 in ...)

I checked this error. And the problem is this piece of code:

if (file_prepare_directory($dir, FILE_CREATE_DIRECTORY)) {
    $file = file_save_data($contents, 'public://js/gmap_markers.js', FILE_EXISTS_REPLACE);
    variable_set('gmap_marker_file', $file->fid);
  }

Apparently $file can be empty and thus not being an object, resulting in the error.

This must be corrected into this:

if (file_prepare_directory($dir, FILE_CREATE_DIRECTORY)) {
    $file = file_save_data($contents, 'public://js/gmap_markers.js', FILE_EXISTS_REPLACE);
    if (!empty($file)) {
      variable_set('gmap_marker_file', $file->fid);
    }
  }