Regression: calling render() before first open throws

[web-padawan]

Description

Calling combobox.render() throws in case combo-box has not yet been open:

Cannot read property 'querySelectorAll' of undefined

This is the line where is happens:

vaadin-combo-box/src/vaadin-combo-box-mixin.html

Line 276 in 41f8709

this.$.overlay._selector.querySelectorAll('vaadin-combo-box-item').forEach(item => item._render());

Expected outcome

Calling combobox.render() doesn't throw

Actual outcome

Error is thrown

Steps to reproduce

1. Put a vaadin-combo-box element in the page.
2. Assign the renderer, don't open combo-box
3. Run combobox.render()

[web-padawan]

Note: this was reported for the dom-if version but should be still valid after disable-upgrade change as _selector isn't defined either until combo-box is opened for the first time.

[web-padawan]

Discussed in person that render() could actually call _initDropdown, if called before first opening of the combobox, so as it would trigger actual rendering as users might expect it to do.

[tomivirkki]

Actual items wouldn't still be rendered before the dropdown is opened so this wouldn't invoke any renderers. From this point of view would make sense to not do anything unless _selector is there.

[Haprog]

After #782 is merged and build is green, I will rebase this fix on latest master to get this one to pass also.

[Haprog]

This fix is now included in v4.2.6