The ETRSi has an interlock that means you should not be able to remove the film back without the darkslide in place. However, you will still be able to open the filmback and therefor ruin some of the shots on the film, if you open the film back mid film. However, there is a good chance that some of the shots will be OK, because the backing paper will be covering the exposed film on the takeup spool (I am assuming that you have 120 film, not 220). Most of the unexposed film on the other spool should also be OK. Opening the back mid-roll will reset the exposure counter and lose you some shots as the mechanism will then be set to take up the paper leader, as if a new roll of film had been inserted.
It is possible to operate the ETRSi without a darkslide, as the film insert is removable with the film back in situ.