We are using a Visualforce page while converting the Lead.
We don't want to send a notification to the owner with the checkbox Send notification to the owner, so we wanted to take it out of the checkbox or make it read-only, but I am getting this error:

Cannot set property sendNotificationEmail to NULL
Error is in expression '{!convertLead}' in component <apex:commandButton> in page leadconvertpage: Class.leadConvertController.convertLead: line 131, column 1

An unexpected error has occurred. Your development organization has been notified.

Here is the screenshot:

We want to send this email notification via workflow rule with a different email template.
Please check and let me know how can I take out the checkbox from the VF page.
Thanks