fix a bug with incorrectly File.Sheets order

I have a xlsx with 3 sheets. sheet1 sheet2 sheet3
First sheet has 7*500 cells.
Second and third sheet is empty.
File.Sheets[0] is empty,File.Sheets[1] is empty, File.Sheets[2] has 7*500 cells.

I think this bug come from the unpredictable gocoroutine execute order. 
Sorry,but I can not add a test to this bug.I can not public that xlsx file.