telerik textbox selectAll() property does not work

telerik textbox selectAll() property does not work

I want to select textbox's text when it got focus. I have found many
links/ answers about "selectAll()" property and it does work. but the
problem is that I have three inputs (a textbox, then combo box, then
textbox) and i want to set selectAll() on 3rd input (means on 2nd textbox)
when i come to that textbox through combobox, it does work but if i make
comboBox disabled, it doesnot work, it just focuses there but does not
select whole text.
I don't know why its happening? Any kind of help will be appreciated.