Unable to parse fhirpath grammar

[AlexKovalevych]

Grammar taken from here: https://github.com/HL7/fhirpath.js/blob/master/src/parser/FHIRPath.g4

Parsed like:

#![feature(try_blocks)]

#[macro_use]
extern crate lazy_static;

mod parser;

use antlr_rust::common_token_stream::CommonTokenStream;
use antlr_rust::input_stream::InputStream;
use antlr_rust::parser_rule_context::ParserRuleContext;
use antlr_rust::tree::{ErrorNode, ParseTreeListener, TerminalNode};

use parser::fhirpathlexer::FHIRPathLexer;
use parser::fhirpathlistener::FHIRPathListener;
use parser::fhirpathparser::*;

struct Listener;

impl ParseTreeListener for Listener {
    fn visit_terminal(&mut self, node: &TerminalNode) {}
    fn visit_error_node(&mut self, node: &ErrorNode) {}
    fn enter_every_rule(&mut self, ctx: &dyn ParserRuleContext) {}
    fn exit_every_rule(&mut self, ctx: &dyn ParserRuleContext) {}
}

impl FHIRPathListener for Listener {
...
}

fn main() {
    let input = String::from("Observation.value");
    let lexer = FHIRPathLexer::new(Box::new(InputStream::new(input)));
    let token_source = CommonTokenStream::new(lexer);
    let mut parser = FHIRPathParser::new(Box::new(token_source));
    parser.add_parse_listener(Box::new(Listener {}));
    let result = parser.expression();
    assert!(result.is_ok());
}

Getting error:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /home/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/antlr-rust-0.1.1/src/parser.rs:442:79
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1076
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:217
  10: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:526
  11: rust_begin_unwind
             at src/libstd/panicking.rs:437
  12: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
  13: core::panicking::panic
             at src/libcore/panicking.rs:50
  14: core::option::Option<T>::unwrap
             at /home/alex/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/macros/mod.rs:10
  15: antlr_rust::parser::BaseParser<Ext,T>::unroll_recursion_context
             at /home/alex/.cargo/registry/src/github.com-1ecc6299db9ec823/antlr-rust-0.1.1/src/parser.rs:442
  16: fhir_server_fhirpath::parser::fhirpathparser::FHIRPathParser::expression_rec
             at src/parser/fhirpathparser.rs:1493
  17: fhir_server_fhirpath::parser::fhirpathparser::FHIRPathParser::expression
             at src/parser/fhirpathparser.rs:1097
  18: fhir_server_fhirpath::main
             at src/main.rs:534
  19: std::rt::lang_start::{{closure}}
             at /home/alex/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:67
  20: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:52
  21: std::panicking::try::do_call
             at src/libstd/panicking.rs:348
  22: std::panicking::try
             at src/libstd/panicking.rs:325
  23: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  24: std::rt::lang_start_internal
             at src/libstd/rt.rs:51
  25: std::rt::lang_start
             at /home/alex/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libstd/rt.rs:67
  26: main
  27: __libc_start_main
  28: _start

rust version: 1.46.0-nightly