UngeheuerLich
Legend
Just for information from the SRD
In no way the trigger has to be an action. It is just not there. So anyone saying the spellcasting is finished is wrong here.
Ready
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.
In no way the trigger has to be an action. It is just not there. So anyone saying the spellcasting is finished is wrong here.