[Bug Fixing] avoid crash when getFlatUIContext() return null

[fred-ye]

First of all, thank you for your contribution!

Please makes sure that these checkboxes are checked before submitting your PR, thank you!

CheckList:

if isBugFix :

• Make sure that you add at least one unit test for the bug which you had fixed.

elif isNewFeature :

• Update API docs for the component.
• Update/Add demo to demonstrate new feature.
• Update test scripts for the feature.
• Add unit tests for the feature.

[weex-bot]

	Fails
🚫	Failed to run assembleDebug task for android.

	Messages
📖	android build verification finished.

Generated by 🚫 dangerJS

[YorkShen]

Use the code style of WXText, thanks

[YorkShen]

LGTM