Fix Jira #ROOT-8474
Fix Jira #ROOT-8474

Add a protection when trying to set text to a non-existant part of the status bar. This fixes Jira #ROOT-8474 Setting status bar of TRootBrowser to 1 column produces error message if a TCanvas is also open.

tc3t/qoot,tc3t/qoot,tc3t/qoot,tc3t/qoot,tc3t/qoot,tc3t/qoot,tc3t/qoot,tc3t/qoot,tc3t/qoot,tc3t/qoot