When a page contains a thumbnail instead of a fullsize image, wfFindFile will not find the file in \DocBookExportAPI::getDocbookfromWikiPage, returning false.
As there is no check for that case, calling getLocalRefPath() on its return path will fail.
Here is the stack trace:
/w/api.php?action=getdocbook&outputformat=pdf&bookname=Test Error from line 326 of /var/www/html/w/extensions/DocBookExport/DocBookExportAPI.php: Call to a member function getLocalRefPath() on boolean
Backtrace:
#0 /var/www/html/w/extensions/DocBookExport/DocBookExportAPI.php(71): DocBookExportAPI->getDocbookfromWikiPage(string, ParserOptions, string, array, array)
#1 /var/www/html/w/includes/api/ApiMain.php(1583): DocBookExportAPI->execute()
#2 /var/www/html/w/includes/api/ApiMain.php(546): ApiMain->executeAction()
#3 /var/www/html/w/includes/api/ApiMain.php(517): ApiMain->executeActionWithErrorHandling()
#4 /var/www/html/w/api.php(94): ApiMain->execute()
#5 {main}