[FIX] sap.m.NavContainer would fail if the current page has been removed
[FIX] sap.m.NavContainer would fail if the current page has been removed

BCP: 1580097536
sap.m.NavContainer assumes there is always a currentPage, but if it is removed in this case due to error handling, thus the NavContainer fails
This fix checks if the oFromPage actually exist before continuing with the transition

Change-Id: I58e9672e5f559fbe95bec9b01ed90faa1a2c2d63

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