DropdownMenuItem from SQFLite using Flutter - flutter

Question

I've seen other similar topics but it didn't help me. I'm not a programmer or have a strong background on this field, just a mechanical engineer trying to develop some apps ;)
So, here's the thing, when I use dropdownmenu, i usually define a list with the items beforehand, like the below:
List<String> varfluid = ["CO2", "O2"];
String _varfluid = "CO2";
void metFluid(String value){
setState((){
_varfluid=value;
});
}
This is defined before, and the below is how I create my DropdownButton
new DropdownButton(
onChanged: (String value){
metFluid(value);
},
value: _varfluid,
items: varfluid.map((String value){
return new DropdownMenuItem(
value: value,
child: new Text(value),
);
}).toList(),
)
It is working fine for me so far, the challenge I'm facing now is to populate another DropDown with items from a sqflite db. I tried to do it in a similar way to the above but I'm getting an error. Here's what I have:
This is the method for returning a list with all the items from the DB
Future<List> pegarTanques() async {
var bdCliente = await db;
var res = await bdCliente.rawQuery("SELECT * FROM $tabelaTanque");
return res.toList();
}
Then I run this piece of code to get only the desired property of the object to my List.
List todosTanques;
List<String> nomestanques = [""];
String _nomestanques = "";
void listartanques() async {
var db = new BD();
todosTanques = await db.pegarTanques();
for (int i = 0; i < todosTanques.length; i++) {
Tanque tanque = Tanque.map(todosTanques[i]);
nomestanques.add(tanque.nome);
}
}
And then I'm getting the list just fine. However when I try to implement it the same way as I did with the dropdown with predefined items, everytime I click on an item from the dropdown list I get this error:
'package:flutter/src/material/dropdown.dart': Failed assertion: line 620 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not true.
User-created ancestor of the error-causing widget was:
MaterialApp file:///C:/Users/Victor/AndroidStudioProjects/formv3/lib/main.dart:9:14
Any help on this?