when add a blog entry to a course, it shows the error below:

Coding error detected, it must be fixed by a programmer:
The theme has already been set up for this page ready for output. Therefore, you can no longer change the theme, or anything that might affect what the current theme is, for example, the course.
More information about this error

Debug info: Stack trace when the theme was set up:

• line 677 of /lib/pagelib.php: call to moodle_page->initialise_theme_and_output()
• line 820 of /lib/pagelib.php: call to moodle_page->magic_get_theme()
• line 344 of /local/edwiserform/lib.php: call to moodle_page->__get()
• line 1422 of /lib/navigationlib.php: call to local_edwiserform_extend_navigation()
• line 3977 of /lib/navigationlib.php: call to global_navigation->initialise()
• line 758 of /lib/pagelib.php: call to settings_navigation->__construct()
• line 820 of /lib/pagelib.php: call to moodle_page->magic_get_settingsnav()
• line 71 of /blog/edit.php: call to moodle_page->__get()
  Error code: codingerror

Stack trace: • line 1918 of /lib/pagelib.php: coding_exception thrown

• line 1001 of /lib/pagelib.php: call to moodle_page->ensure_theme_not_set()
• line 2699 of /lib/moodlelib.php: call to moodle_page->set_course()
• line 75 of /blog/edit.php: call to require_login()