throw error while generating type and mutation base on mongoose schema
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### Current Behavior
<!--- If describing a bug, tell us what happens instead of the expected behavior.
      Provide a screenshot when possible and full code sample -->
<!--- If suggesting a change/improvement, explain the difference from current behavior -->
when run the command `create-graphql generate -tm Post --schema Post`, throw the error

<details>
 <summary>error</summary>

```
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.<anonymous> (/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.<anonymous> (/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.<anonymous> (/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.<anonymous> (/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)
```
</details>

<details>
 <summary>Post.js</summary>

```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);
```
</details>

### Expected Behavior
<!--- If you're describing a bug, tell us what should happen -->
<!--- If you're suggesting a change/improvement, tell us how it should work -->

### Your Environment
<!--- Include as many relevant details about the environment you experienced the bug in -->

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