The film does not come in contact with bubbles because it is protected. It is intraoral, meaning it is designed to be inserted into your mouth, which is wet. It does not come in contact with solids, either. Other x-ray detectors are also protected, and have been used, as I mentioned.
The plastic cover on the film is highly permeable to hydrogen. The hypering of regular film occurs through the plastic support of the silver nitrate crystals that when developed give the image. The bubbles are either pure of mixed hydrogen, which will adhere to the plastic and diffuse throughout the piece of film.