How to get information from another script

Hi guys, i am new to this site and i already found alot. Wich i apreciate ! But I have a question about using another script into a script. Im not sure about the correct way to do this. This is what i have yet ;

I have a script that make a countdown timer, and i want to making things about in relation with this timer. Is this a good way to write it ? Becuase i have a warning every frame : Unassigned Reference.

if ( (timer as CountDownTmrScript).countDownSeconds == 0 && NumberEnemies == 0 && !NewWave)
NewWave = true;
WaveNumber += 1;

Thanks for your help !

You need find to gameobject with contain “countdowm timer” script then find to"countdowm timer" script into gameobject.