throw error while generating type and mutation base on mongoose schema

### Current Behavior

when run the command `create-graphql generate -tm Post --schema Post`, throw the error

error

```
events.js:167
throw er; // Unhandled 'error' event
^

TypeError: Cannot read property 'name' of undefined
at Context.visitExpression (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/generators/utils.js:317:63)
at Context.invokeVisitorMethod (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:344:49)
at Visitor.PVp.visitWithoutReset (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:196:32)
at visitChildren (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:246:25)
at Visitor.PVp.visitWithoutReset (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:204:20)
at NodePath.each (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path.js:101:26)
at visitChildren (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:219:18)
at Visitor.PVp.visitWithoutReset (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:204:20)
at visitChildren (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:246:25)
at Visitor.PVp.visitWithoutReset (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:204:20)
Emitted 'error' event at:
at Immediate. (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/yeoman-generator/lib/index.js:424:16)
at runCallback (timers.js:696:18)
at tryOnImmediate (timers.js:667:5)
at processImmediate (timers.js:649:5)
events.js:167
throw er; // Unhandled 'error' event
^

TypeError: Cannot read property 'name' of undefined
at Context.visitExpression (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/generators/utils.js:317:63)
at Context.invokeVisitorMethod (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:344:49)
at Visitor.PVp.visitWithoutReset (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:196:32)
at visitChildren (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:246:25)
at Visitor.PVp.visitWithoutReset (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:204:20)
at NodePath.each (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path.js:101:26)
at visitChildren (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:219:18)
at Visitor.PVp.visitWithoutReset (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:204:20)
at visitChildren (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:246:25)
at Visitor.PVp.visitWithoutReset (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/ast-types/lib/path-visitor.js:204:20)
Emitted 'error' event at:
at Immediate. (/home/bakhtiyor/.npm-global/lib/node_modules/generator-graphql/node_modules/yeoman-generator/lib/index.js:424:16)
at runCallback (timers.js:696:18)
at tryOnImmediate (timers.js:667:5)
at processImmediate (timers.js:649:5)
(node:20340) UnhandledPromiseRejectionWarning: Error: Exited with status 1
at closeArgsToError (/home/bakhtiyor/.npm-global/lib/node_modules/create-graphql/node_modules/cross-spawn-promise/lib/index.js:20:16)
at ChildProcess. (/home/bakhtiyor/.npm-global/lib/node_modules/create-graphql/node_modules/cross-spawn-promise/lib/index.js:76:19)
at Object.onceWrapper (events.js:273:13)
at ChildProcess.emit (events.js:182:13)
at Process.ChildProcess._handle.onexit (internal/child_process.js:237:12)
(node:20340) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:20340) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
(node:20340) UnhandledPromiseRejectionWarning: Error: Exited with status 1
at closeArgsToError (/home/bakhtiyor/.npm-global/lib/node_modules/create-graphql/node_modules/cross-spawn-promise/lib/index.js:20:16)
at ChildProcess. (/home/bakhtiyor/.npm-global/lib/node_modules/create-graphql/node_modules/cross-spawn-promise/lib/index.js:76:19)
at Object.onceWrapper (events.js:273:13)
at ChildProcess.emit (events.js:182:13)
at Process.ChildProcess._handle.onexit (internal/child_process.js:237:12)
(node:20340) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
```

Post.js

```js
'use strict'

const mongoose = require('mongoose');
const Schema = mongoose.Schema;

const PostSchema = new Schema({
title: {
type: String,
required: true
},

body: {
type: String,
required: true
}
});

module.exports = mongoose.model('Post', PostSchema);
```

### Expected Behavior

### Your Environment

| software | version
| ---------------- | -------
| create-graphql | 1.2.3
| node | v10.6.0
| npm or yarn | 6.4.1