Flutter working with nested Dictionary/Map

I am trying to get my data out of a dictionary, but somehow nothing works.

I have the following dictionary structure:

class TaskTags {
  var tagDict = <String, Object>{};
  TaskTags({
    this.tagDict,
  });

  factory TaskTags.fromJson(Map<String, dynamic> json){
    var innerMap = json['tagDict'];
    var tagMap = Map<String, TaskTag>();
    innerMap.forEach((key, value) {
      tagMap.addAll({key: TaskTag.fromJson(value)});
    });
    return new TaskTags(
      tagDict: tagMap,
    );
  }
}

class TaskTag {
  String helpHdr;
  String helpText;
  TaskTag({
    this.helpHdr,
    this.helpText,
});

  factory TaskTag.fromJson(Map<String, dynamic> json){
    return new TaskTag(
      helpHdr: json['helpHdr'],
      helpText: json['helpText'],
    );
  }
}

This matches to JSON data such as (where abc and def are dictionary keys)

{
  "tagDict" : {
    "abc" : {
      "helpHdr" : "short text1",
      "helpText" : "long text1"
    },
    "def" : {
      "helpHdr" : "short text2",
      "helpText" : "long text2"
    }
  }
}

This is stored in a variable listOfTags. In the debugger, it has a structure of

listOfTags
  tagDict
    0:key="abc"
      value=  helpHdr = "..."
              helpText = "..."
    1:  key="def"
      value=  helpHdr = "..."
              helpText = "..."

Now I wanted to get the data out with

header: listOfTags.tagDict["abc"].helpHdr,

Doesn't work, I also get no appropriate suggestions when typing the . after the ]. I guess, I have to approach this slightly different - but how?