The builder returned by ConvertLambda hands back null when a nested
conversion declines, and the result was cast and dereferenced before
anything checked it, so a tree the transform cannot handle took down the
whole method with a NullReferenceException instead of being left alone.
EF Core's StringCharConverter.ToChar is such a tree: the conditional
spills the Expression.Call arguments into stack slots, which
MatchGetMethodFromHandle does not see through.
Assisted-by: Claude:claude-opus-5:Claude Code