[INTERNAL][FIX] uxap.ObjectPageLayout: _getFirstVisibleSubSection behavior fixed
[INTERNAL][FIX] uxap.ObjectPageLayout: _getFirstVisibleSubSection
behavior fixed

BCP: 1870038109

Problem: _storeScrollLocation is called when the page is not loaded yet

Solution: Ensure ObjectPageLayout has a dom reference when calling the
function

Change-Id: I8af7318eb4b72726528b1e7df821c5a90dca71cf

cschuff/openui5,SAP/openui5,SAP/openui5,cschuff/openui5,cschuff/openui5,SAP/openui5,SAP/openui5