If you make a section of elevated track transparent, then destroy it by deconstructing the rail support(s) connected to it, the game will throw this error when the mod tries to made the rails opaque again:

The mod Transparent elevated rails (0.2.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event transparent-elevated-rails::on_tick (ID 0)
LuaEntity API call when LuaEntity was invalid.
stack traceback:
    [C]: in function '__index'
    __transparent-elevated-rails__/control.lua:107: in function <__transparent-elevated-rails__/control.lua:101>

This error also occurs if you use the hover mode to make an elevated rail transparent, then use the manual tool to make it opaque again before it automatically restores itself.

Thanks for this report! I fixed it in the latest version.