You're not setting a source image for the FilterEffect, and instead passing the source stream directly into the target WriteableBitmap for some reason.
When you kick off the RenderAsync operation, the FilterEffect Source property is null, and this is what causes the exception. You should pass an image source into either the FilterEffect constructor, or set its Source property.
I recommend a StreamImageSource since you've got a System.IO.Stream with the image data.
Conceptually, this is how to think of it:
chosen photo stream -> StreamImageSource -> FilterEffect -> WriteableBitmapRenderer -> the writeable bitmap
And more concretely:
using(var streamSource = new StreamImageSource(stream))
using(var filterEffect = new FilterEffect(streamSource, filters))
using(var writeableBitmapRenderer = new WriteableBitmapRenderer(filterEffect, writeableBitmap))
{
await writeableBitmapRenderer.RenderAsync();
....
}