Based on the error message the
workbook
is null when the finally block is executed. So what probably happens is that previously in your try or catch block you either don't set the workbook at all or you set it to null in some place.
Without seeing te whole code, it's impossible to say, but could it be that in case of an error for example when opening a workbook, you hit the catch block and when finally block is entered, the variable hasn't yet been set.
You should investigate the source of the problem but in order to handle such situations you should also add
finally
{
if (workbook != null) {
workbook.Close(Missing.Value, Missing.Value, Missing.Value);
}
if (application != null) {
application.Quit();
}
}