The only required properties of a Canopy object in a minecraft:tree_feature are the leaf_block and a canopy_offset object with "min" and "max" values that apparently can be float values. I would assume positive integers are intended, but positive, negative, float values, and any mixture thereof will cause Minecraft to immediately crash when loading.

Steps to reproduce:

Include the attached feature and feature_rule file in the "feature" and "feature rule" folders of a given behavior pack and proceed to any birch biome (ex: seed 499819143 and coordinates around -400, 90 -800).

What happens:

Minecraft begins to load and then abruptly crashes. (If you teleport you'll get the sky, paper doll, and coordinates before it does).

What I expect to happen:

Minecraft should generate a couple of weird looking trees throughout the Birch biome with grass trunks and a podzol canopy.