[FIX] sap.m.Popover: Fix failing qUnit tests.
[FIX] sap.m.Popover: Fix failing qUnit tests.

Some tests were failing under IE. The issue is that in some cases
the document.activeElement is null-ed and there's an exception in the
popover.

BCP: 1780146290

Change-Id: I20ddefe3e84a6fa15aac9b0c5ac743fc09af4022

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