segfault when try to run export_srcpkg example

Hi. I am trying to run the example here https://tl2cgen.readthedocs.io/en/latest/treelite-migration.html but run into segfault when try to export the source package.

Here is my python code, essentially just copied from the link:

```
import treelite
import tl2cgen

builder = treelite.ModelBuilder(num_feature=3)

tree = treelite.ModelBuilder.Tree()
tree[0].set_numerical_test_node(
feature_id=0,
opname="<",
threshold=5.0,
default_left=True,
left_child_key=1,
right_child_key=2
)
tree[1].set_leaf_node(0.6)
tree[2].set_leaf_node(-0.4)
tree[0].set_root()

builder.append(tree)
model = builder.commit() # Obtain treelite.Model object

tl2cgen.export_srcpkg(model, toolchain='gcc', pkgpath='./mymodel_pkg.zip', libname = "./mymodel.so", params={})
```

Here is the bt from the gdb:

```
#0 0x00007fffe76baf86 in void rapidjson::GenericReader, rapidjson::UTF8, rapidjson::CrtAllocator>::ParseString<0u, rapidjson::GenericStringStream >, rapidjson::GenericDocument, rapidjson::MemoryPoolAllocator, rapidjson::CrtAllocator> >(rapidjson::GenericStringStream >&, rapidjson::GenericDocument, rapidjson::MemoryPoolAllocator, rapidjson::CrtAllocator>&, bool) ()
from /home/python/py3.10/lib/libtreelite.so
#1 0x00007fffe76bc160 in void rapidjson::GenericReader, rapidjson::UTF8, rapidjson::CrtAllocator>::ParseValue<0u, rapidjson::GenericStringStream >, rapidjson::GenericDocument, rapidjson::MemoryPoolAllocator, rapidjson::CrtAllocator> >(rapidjson::GenericStringStream >&, rapidjson::GenericDocument, rapidjson::MemoryPoolAllocator, rapidjson::CrtAllocator>&) ()
from /home/python/py3.10/lib/libtreelite.so
#2 0x00007fffe5de2b68 in tl2cgen::compiler::CompilerParam::ParseFromJSON(char const*) () from /home/python/py3.10/lib/libtl2cgen.so
#3 0x00007fffe5ddde52 in TL2cgenGenerateCode () from /home/python/py3.10/lib/libtl2cgen.so
#4 0x00007ffff7e3d052 in ffi_call_unix64 () from /home/python/py3.10/lib/python3.10/lib-dynload/../../libffi.so.8
```

Wonder if anyone might have insight about what happened. Thanks!