Thank your lucky stars that only the lens popped off. Mine tend to get snapped at the base and just hang there. My BX is used to mow narrow orchards.
Before that, I tried putting a couple of drops of hot melt glue on the lens to keep it in place.
Ultimately, I decided that, since the tractor never goes on the road and is not used at night, the turn signal and emergency blinker are not necessary so are better just left off.
This, compounded with the dash switches being a complete pain when they get wet, led me to abandon the lights. The switches can just be popped out and unplugged.