Exception When Creating Folder

I'm trying to save a bunch of files into a folder. That folder will already exist, but I want to completely delete it and its contents and save the new files into the folder. I'm using the following code:

Project project;        //This is a parameter handed in when the method is called
List<InkStrokeContainer> containers;      //This is a parameter handed in when the method is called

var projectFolder = await ApplicationData.Current.LocalFolder.GetFolderAsync(project.Id.ToString());
var slidesFolder = await projectFolder.CreateFolderAsync("Slides", CreationCollisionOption.ReplaceExisting);

StorageFile file;
IRandomAccessStream stream;

for (int i = 0; i < containers.Count; i++)
{
    if (containers[i].GetStrokes().Count > 0)
    {
        file = await slidesFolder.CreateFileAsync($"{i.ToString()}.jpg", CreationCollisionOption.ReplaceExisting);
        stream = await file.OpenAsync(FileAccessMode.ReadWrite);
        await containers[i].SaveAsync(stream);
        await stream.FlushAsync();
    }
}

However, I'm getting an exception the second time I try to do this:

Cannot create a file when that file already exists. (Exception from HRESULT: 0x800700B7)

Note that I can save once and it replaces the folder correctly. It's only when I try to do it a second time that it fails. It should be replacing the existing item on a collision so I suspect that I'm not releasing something correctly, but I've no idea what.

Ah I found it! I replaced the stream-saving code with this:

using (var stream = await file.OpenAsync(FileAccessMode.ReadWrite))
    await containers[i].SaveAsync(stream);

and it now works fine.

Replacing await stream.FlushAsync() with stream.Dispose() also seems to do the trick.