How can I avoid an error: No such environment variable?

In my code I am using environment variables, but if it (env.var) doesn't exist, I get the error message NAME_ENV_VAR: no such variable, and my script stops executing. For example, in the line

 myeval $env($File)

I receive an error:

 can't read "env(NIKE_TECH_DIR)": no such variable
    while executing
"myeval $env($File)"
    (procedure "chooseRelevantFiles" line 39)
    invoked from within
"chooseRelevantFiles $::GlobalVars::reqStage"
(file "/vobs/tavor/src/Scripts/ReproduceBug.tcl" line 575)

How can I avoid this error and go on to execute my script?