Just set _bitmap into null
If concerning dispose the _bitmap will affect ToSKBitmap().
I suggest to change ToSKBitmap() to:
public SKBitmap ToSKBitmap()
{
return _bitmap.Copy();
}
or
// Developer should be responsible for disposing an 'Underlying' object which might cause some problems
public SKBitmap GetUnderlyingSKBitmap()
{
return _bitmap;
}