[Flutter ]Unhandled Exception: NoSuchMethodError: The method '[]' was called on null

Question

class Resistencia100{
  int id;
  double r_pos1;
  double r_pos2;
  double r_pos3;
  double r_pos4;
  double r_pos5;

  Resistencia100({
    this.id, this.r_pos1, this.r_pos2, this.r_pos3, this.r_pos4,
    this.r_pos5
    });

  Map<String, dynamic> toMap() => {
    "id": id,
    "r_pos1": r_pos1,
    "r_pos2": r_pos2,
    "r_pos3": r_pos3,
    "r_pos4": r_pos4,
    "r_pos5": r_pos5,

  };
  factory Resistencia100.fromMap(Map<String, dynamic> json) => new Resistencia100(
    id:     json["id"],
    r_pos1: json["r_pos1"],
    r_pos2: json["r_pos2"],
    r_pos3: json["r_pos3"],
    r_pos4: json["r_pos4"],
    r_pos5: json["r_pos5"],
  );

}

This is my Model class Resistencia100, Now we will see how I request the data through my get method

  Future<List<Resistencia100>> getAllResistencia100() async {
    final db = await database;
    var response = await db.query("Resistencia100");
    List<Resistencia100> list = response.map((c) => Resistencia100.fromMap(c)).toList();
    print("Cantidad ID: "+list[0].id.toString());
    print("Cantidad r_pos1: "+list[0].r_pos1.toString());
    print("Cantidad r_pos2: "+list[0].r_pos2.toString());
    print("Cantidad r_pos3: "+list[0].r_pos3.toString());
    print("Cantidad r_pos4: "+list[0].r_pos4.toString());
    print("Cantidad r_pos5: "+list[0].r_pos5.toString());
    return list;
  }

The information is coming correctly to the method, now I try to extract that information and the error is coming.

List <Resistencia100> resistencia100 = new List<Resistencia100>();

Future<List<Resistencia100>> getResistencia100() async {
    await ClientDatabaseProvider.db.getAllResistencia100();
  }

  void validate() async {

    resistencia100 = await getResistencia100();

    print("RESISTENCIA ID: "+resistencia100[0].id.toString());


  }

The truth is that I don't understand the reason for the error very well, I hope you can understand, I will leave the textual error in the following lines, this is generated in the "print".

[ERROR:flutter/lib/ui/ui_dart_state.cc(148)] Unhandled Exception: NoSuchMethodError: The method '[]' was called on null.
Receiver: null
Tried calling: [](0)
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:51:5)
#1      _ConfigConcretoState.validate (package:entremuros/vistas/configconcreto.dart:282:44)

Solution

Your method getResistencia100() is not returning anything. So at validate() your variable resistencia100 is transforming into a null after await the getResistencia100()

A solution is change the getResistencia100(), adding a return statement

Future<List<Resistencia100>> getResistencia100() async {
    return await ClientDatabaseProvider.db.getAllResistencia100();
}