What I generally do with these kind of issues is to isolate which object is the problem, I start by adding another part of the hierarchy until I get the issue. I then look at debugging that issue. So I would do something like:
If Browser(oBrowser).Exist Then
MSgBox "Part-1 OK"
If Browser(oBrowser).FlexApplication(oFlexApp).Exist Then
MSgBox "Part-2 OK"
If Browser(oBrowser).FlexApplication(oFlexApp).FlexCo ntainer(oFlexContainer).Exist Then
MSgBox "Part-3 OK"