Since a couple of weeks, some of my users have a problem with a custom Javascript button in Classic. They get the error message "Unable to set property 'innerHTML' of undefined or null reference". No line number is reported.

The error does not always occur, and after trying two or three times the button does what it is supposed to do, no error message. The used browser is Chrome. I cannot reproduce this message, I clicked dozens of times and for me, the button always works.

This is the code:
 

{!REQUIRESCRIPT("/soap/ajax/37.0/connection.js")} {!REQUIRESCRIPT("/soap/ajax/37.0/apex.js")} var pdfOverlay = QuotePDFPreview.quotePDFObjs['quotePDFOverlay']; try{ var verkoopGroep = "{!Quote.Account_verkoopgroep__c}"; if(verkoopGroep.indexOf("F0") == 0) { pdfOverlay.summlid = '0EH580000003OAC'; } else { var taal = "{!Quote.Account_taal__c}"; switch(taal){ case 'Nederlands': pdfOverlay.summlid = '0EH58000000GcPS';break; case 'Engels': pdfOverlay.summlid = '0EH58000000GjHG';break; case 'Duits': pdfOverlay.summlid = '0EH58000000Gg9F';break; case 'Spaans': pdfOverlay.summlid = '0EH5800000085ZN';break; case 'Roemeens': pdfOverlay.summlid = '0EH5800000085ZS';break; case 'Frans': pdfOverlay.summlid = '0EH5800000085ZX';break; case 'Pools': pdfOverlay.summlid = '0EH5800000085Zh';break; case 'Italiaans': pdfOverlay.summlid = '0EH5800000085Zc';break; default:pdfOverlay.summlid = '0EH58000000GjHG';break; } } } catch(e){ alert('Er is een fout opgetreden: '+e); pdfOverlay.summlid = '0EH58000000GcPS'; }; pdfOverlay.dialog.buttonContents = '<input value=\"{!$Label.Opslaan_naar_offerte}\" class=\"btn\" name=\"save\" onclick=\"QuotePDFPreview.getQuotePDFObject(\'quotePDFOverlay\').savePDF(\'0\',\'0\');\" title=\"Opslaan naar offerte\" type=\"button\" /><input value=\"{!$Label.Offerte_opslaan_en_per_e_mail_verzenden}\" class=\"btn\" name=\"saveAndEmail\" onclick=\"QuotePDFPreview.getQuotePDFObject(\'quotePDFOverlay\').savePDF(\'1\');\" title=\"Offerte opslaan en per e-mail verzenden\" type=\"button\" /><input value=\"{!$Label.Annuleren}\" class=\"btn\" name=\"cancel\" onclick=\"QuotePDFPreview.getQuotePDFObject(\'quotePDFOverlay\').close();\" title=\"Annuleren\" type=\"button\" />'; pdfOverlay.setSavable(true); pdfOverlay.setContents('/quote/quoteTemplateDataViewer.apexp?id={!Quote.Id}','/quote/quoteTemplateHeaderData.apexp?id={!Quote.Id}'); pdfOverlay.display();

It creates a PDF for a quote and displays it. This code has worked properly for the past 3 years, but all of a sudden some of my users have problems with it.

Note that 'innerHTML' does not occur in the Javascript, so the problem occurs in the QuotePDFPreview library?