Dockerfile's resolveVariable(*) throws TypeError if the line number is invalid

[rcjsuen]

import { DockerfileParser } from './main'
let dockerfile = DockerfileParser.parse("FROM alpine");
dockerfile.resolveVariable("image", 1);

$ node out/src/test.js
dockerfile-ast/out/src/dockerfile.js:107
        let resolvedVariable = image.resolveVariable(variable, line);
                                     ^

TypeError: Cannot read property 'resolveVariable' of null
    at Dockerfile.resolveVariable (dockerfile-ast/out/src/dockerfile.js:107:38)
    at Object.<anonymous> (dockerfile-ast/out/src/test.js:5:12)
    at Module._compile (module.js:653:30)
    at Object.Module._extensions..js (module.js:664:10)
    at Module.load (module.js:566:32)
    at tryModuleLoad (module.js:506:12)
    at Function.Module._load (module.js:498:3)
    at Function.Module.runMain (module.js:694:10)
    at startup (bootstrap_node.js:204:16)
    at bootstrap_node.js:625:3