Internal error

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[3983c082a604e3c728a27504] /w/index.php?title=Calculations:Earthing_Calculation&action=pdfbook Error: Call to a member function getFreshParser() on null

Backtrace:

from /home/fileng/web/filipinoengineer.com/public_html/w/extensions/PdfBook/PdfBookAction.php(21)
#0 /home/fileng/web/filipinoengineer.com/public_html/w/includes/MediaWiki.php(542): PdfBookAction->show()
#1 /home/fileng/web/filipinoengineer.com/public_html/w/includes/MediaWiki.php(322): MediaWiki->performAction()
#2 /home/fileng/web/filipinoengineer.com/public_html/w/includes/MediaWiki.php(904): MediaWiki->performRequest()
#3 /home/fileng/web/filipinoengineer.com/public_html/w/includes/MediaWiki.php(562): MediaWiki->main()
#4 /home/fileng/web/filipinoengineer.com/public_html/w/index.php(50): MediaWiki->run()
#5 /home/fileng/web/filipinoengineer.com/public_html/w/index.php(46): wfIndexMain()
#6 {main}