Access ID of user from a given map on click of dropdown - Flutter

Question

I had a map given below. The task is to create a dropdown with values showing in the dropdown and when selecting those values I get to store keys in a list. On Selecting the name of the user. The ID must be selected and stored in a list. Tried using the below approach. But it only matches with the First occurrence of the user and returns its ID.

 var idOfUser = groupsController
                                            .userMap.keys
                                            .firstWhere((element) =>
                                                groupsController
                                                        .userMap[element] ==
                                                    value);

**This is User Map ---> {1: Royal Coders, 5: Harshit Gupta, 7: Harshit Gupta, 8: Harshit Gupta, 9: Harshit Gupta, 10: Harshit Gupta, 11: Harshit Gupta, 12: Harshit Gupta, 13: Vinove Coders, 14: Value Coders, 15: Harshit Gupta, 16: Abhay Sharma, 17: SHUBHAM SINGH, 18: Abhay Sharma, 19: Abhay Sharma, 20: Abhay Sharma, 21: Abhay Sharma, 22: Abhay Sharma, 23: Abhay Sharma, 24: Test User, 25: Abhay Sharma, 26: Abhay Sharma, 27: Abhay Sharma, 28: Abhay Sharma, 29: Abhay Sharma, 30: Abhay Sharma, 31: Abhay Sharma, 32: Abhay Sharma, 33: Abh sh, 34: Abh sh, 35: abh vjh, 36: abhay sharma, 37: gdgb cvh, 38: sparsh tandon, 39: sparsh tandon, 40: sparsh tandon, 41: vcghfhh bfghfh, 42: aaaddgh saddgh, 43: SHUBHAM SINGH, 44: 1 string, 45: Abhay Sharma, 46: Sparsh Vinove , 47: 1 string, 48: 1 string, 49: 1 string, 50: Harshit Gupta, 51: Harshit Gupta, 52: Harshit Gupta, 53: Harshit Gupta, 54: 1 @, 55: Harshit Gupta, 56: Harshit Gupta, 57: Harshit Gupta, 58: Harshit Gupta, 59: Harshit Gupta, 60: Harshit Gupta}