Adding Flutter App to F-Droid

Hello F-Droid Community,
I just finished my first App called NextNotes, I just build the apk´s and went from Alpha to Beta to a release ready state by now. It is build with Flutter.
I always released via GitHub but got several requests to bring this App to F-Droid. I tried to write the MetaData file myself, the syntax is ok but when I try to build the App it will always fail.I followed this guide but when building with fdroid build -v -l app.id I will always recieve this error: fdroidserver.exception.VCSException: Git reset failed ==== detail begin ==== fatal: No url found for submodule path 'android/build/com.example.notes' in .gitmodules

It looks like something is wrong with the submodules, however I do not used any.

Should I delete the Build folder and try again?
I got fastlane installed in this Android Project, could this cause harm? ( I would like to move to a automated build process)
What can I try next?

In case you wondered here is the Project link to NextNotes

This is what my app.id.yml file looks like:
Categories: Writing
License: Apache-2.0
SourceCode: https://github.com/daehruoydeef/NextNotes/tree/master/android
IssueTracker: https://github.com/daehruoydeef/NextNotes/issues
LiberapayID: ‘1577341’
Name: NextNotes
Summary: (A free, modern notes app that syncs with Nextcloud Notes)
Description: |4-

NextNotes is a modern Note taking App that syncs with Nextcloud Notes.
It features Offline support, a dark theme and basic Note taking functionality.

Features
* Out of the Box Nextcloud Notes Sync
* Dark / Light Theme
* add Notes
* edit and update Notes
* delete Notes
* mark note as Favourite

RepoType: git
Repo: https://github.com/daehruoydeef/NextNotes.git

Builds:
  - versionName: '1.0'
versionCode: 1
commit: v1.0
subdir: app
gradle:
  - yes
output: NextNotes.apk

AutoUpdateMode: None
UpdateCheckMode: None
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