Unity async task can't be called outside of main thread? - unity3d

Question

I need to convert a .txt file called TheFile to a dictionary called theFileDictionary and then save it to a binary file.
I am doing this with the following class FileHandler:
public class FileHandler : MonoBehaviour
{
[SerializeField] TextAsset theFile;
FileStream file;
public async Task LoadTheFileIntoBinaryFile()
{
BinaryFormatter formatter = new();
file = File.Create(Application.persistentDataPath + "/foods.txt");
/*line 33, error gets called*/ await Task.Run(() => { formatter.Serialize(file, LoadTheFileIntoDictionary()); } );
file.Close();
}
Dictionary<string, string[]> LoadTheFileIntoDictionary()
{
/*line 39, error occurs*/Dictionary<string, string[]> theFileDictionary =
theFile
.text
.Split('\n')
.Select(x => x.Split('\t'))
.ToDictionary(x => x[ApplicationInfo.GetLanguageIndex()].Replace("-", " "));
return blsDictionary;
}
}
When starting LoadTheFileIntoBinaryFile() I get this error:
UnityException: get_bytes can only be called from the main thread.
Constructors and field initializers will be executed from the loading thread when loading a scene.
Don't use this function in the constructor or field initializers, instead move initialization code to the Awake or Start function.
UnityEngine.TextAsset.get_text () (at /Users/bokken/buildslave/unity/build/Runtime/Export/Scripting/TextAsset.cs:23)
FileHandler.LoadTheFileIntoDictionary () (at Assets/Scripts/Daten/FileHandler.cs:39)
FileHandler+<>c__DisplayClass3_0.b__0 () (at Assets/Scripts/Daten/FileHandler.cs:33)
System.Threading.Tasks.Task.InnerInvoke () (at :0)
System.Threading.Tasks.Task.Execute () (at :0)
--- End of stack trace from previous location where exception was thrown ---
From what the error says it seems to be that the Dictionary has to be initialised in Awake() or Start(). When I do that it kinda works but I don't need the method in Start(), I need it sometimes later (and I still need it to load asynchronously)...
I tried declaring and implementing the Dictionary<string, string[]> in Start() but with no change in result.
What am I doing wrong?