What are the steps to reproduce your issue ?

    1. Install 'Manufacture' and enable 'Work Order' in settings
    2. Create product 'ProductA', enable 'Manufacture' in 'Inventory' tab
    3. Create a BOM for 'ProductA'
    4. For that BOM, create a new operation called 'OperationA' with PDF worksheet but don't attach any PDF
    4. Go to 'Manufacture' and create a MO with 'ProductA', the 'OperationA' is automatically set in 'Work Orders'
    5. Start the work order

What is currently happening ?

    When you try to pause, an error is raised.
    'TypeError: Cannot read property 'pdfViewer' of undefined'

What are you expecting to happen ?

    Start and pause the work order even without attached PDF.

Why is this happening ?

    'this.pdfViewer = existing.data('PDFViewerApplication').pdfViewer'
    When there is no pdf attached. The data 'PDFViewerApplication' does not exist.

How to fix the bug ?

    Change worksheet default value to text, and make the field mandatory in case of a PDF type

opw-2387428

closes odoo/odoo#62319

Related: odoo/enterprise#14973
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>