onSubmitInvoiceData = () => {
    const { files } = this.state;
    if(files.length < 1) this.setState({ isLoading: false, errors: {...this.state.errors, general: ['Gelieve minstens één factuur te uploaden']} });
        else {
        const promises = [];
        const filesArr = [];

        files.forEach(file => {
            const uploadTask = Storage.ref(`session-123/files/${file.name}`).put(file);
            promises.push(uploadTask);

            uploadTask.on('state_changed', snapshot => {
                const progress = (snapshot.bytesTransferred / snapshot.totalBytes) * 100;
            }, error => { console.log(error) }, async () => {
                await uploadTask.snapshot.ref.getDownloadURL().then(downloadURL => {
                    console.log('url: ', downloadURL);
                    filesArr.push({id: file.id, name: file.name, url: downloadURL});
                });
            });
        });

        Promise.all(promises).then(tasks => {
            console.log('tasks: ', tasks);
            Database.ref(`sessions/123`).update({
                lastUpdate: Firebase.database.ServerValue.TIMESTAMP,
                files: filesArr
            }).then(() => this.setState({ isLoading: false }))
        });
    }
}