Alternative steps to select all 'Flowed Text' objects (empty or not):
1) open 'Edit > Find…'
2) enter 'flowRoot' into the 'ID:' field
3) search
The resulting selection contains all 'Flowed text' objects which then can be deleted or converted to regular text (shift-click those you want to keep as 'Flowed Text' to remove them from the selection).
Alternative steps to select all 'Flowed Text' objects (empty or not):
1) open 'Edit > Find…'
2) enter 'flowRoot' into the 'ID:' field
3) search
The resulting selection contains all 'Flowed text' objects which then can be deleted or converted to regular text (shift-click those you want to keep as 'Flowed Text' to remove them from the selection).