Add a null check to TMPro sprite asset material checking

[Oskiii]

If the Sprite Asset has not been set in TMPro settings, there is an error which prevents UIEffect from drawing its TMPro warning. This pull request adds a null-check to prevent such error.

Reproducing bug:

• Use UIEffect normally, but make sure that the SpriteAsset field in TMPro Settings is empty
• Selecting the text object with UIEffect causes an error

[mob-sakai]

Hi @Oskiii
Thank you for your PR!