I guess GR is also needed to accurately coordinate space flight, but I highly doubt without GR the space shuttle will have any problem finding its way back home. Gravitational lensing doesn't count as practical application. Maybe a subject with even fewer applications is quantum chromodynamics. Even though QCD is in principle needed to describe nuclear physics, effective theories such as nuclear shell model and liquid droplet model have guided scientists well enough to detonate a nuclear bomb in 1945, well before the completion of QCD in the 1970s. It seems to me that by the time QCD comes out, nuclear physics is already more or less finished. Or I could be wrong.