[FIX] Calendar: Update month picker only if rendered
[FIX] Calendar: Update month picker only if rendered

Prevent exception when onThemeChanged is called and the calendar has not
yet been rendered.

Change-Id: Ie7f671368664e0404823551f60b5b1724e1791cb

olirogers/openui5,nzamani/openui5,SQCLabs/openui5,SAP/openui5,cschuff/openui5,SAP/openui5,nzamani/openui5,nzamani/openui5,olirogers/openui5,cschuff/openui5,SQCLabs/openui5,olirogers/openui5,SQCLabs/openui5,SAP/openui5,cschuff/openui5,SAP/openui5