Non-existant file gives 'length' of undefined error

[SebastienGllmt]

repro

{
  "scripts": {
    "flowgen": "flowgen fileThatDoesNotExist.d.ts"
  },
  "dependencies": {
  },
  "devDependencies": {
    "flowgen": "^1.10.0",
    "typescript": "3.8.3"
  }
}

running npm run flowgen results in this error

(node:18282) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'length' of undefined
    at createSourceFile (/home/proj/node_modules/typescript/lib/typescript.js:19298:110)
    at parseSourceFileWorker (/home/proj/node_modules/typescript/lib/typescript.js:19231:26)
    at Object.parseSourceFile (/home/proj/node_modules/typescript/lib/typescript.js:19102:26)
    at createSourceFile (/home/proj/node_modules/typescript/lib/typescript.js:18925:29)
    at Object.compilerHost.getSourceFile (/home/proj/node_modules/flowgen/lib/cli/compiler.js:106:42)
    at findSourceFile (/home/proj/node_modules/typescript/lib/typescript.js:99293:29)
    at /home/proj/node_modules/typescript/lib/typescript.js:99170:85
    at getSourceFileFromReferenceWorker (/home/proj/node_modules/typescript/lib/typescript.js:99137:34)
    at processSourceFile (/home/proj/node_modules/typescript/lib/typescript.js:99170:13)
    at processRootFile (/home/proj/node_modules/typescript/lib/typescript.js:98993:13)

Should probably be a user-friendly error saying no file was found with that name