Android Studio-TextView显示在预览上，但不显示模拟器/设备

我开始写《面向程序员的Android 6:一种应用程序驱动的方法》一书，遇到了一个问题.我遵循其中一个应用程序的分步方法，但遇到了一个令人沮丧的错误，我无法修复.

I started the "Android 6 for Programmers: An App-Driven Approach" book and ran into a problem. I was following the step-by-step approach for one of the apps and got a pretty frustrating error that I cannot fix.

我添加了文本和图像视图，并在预览"部分以应有的方式显示了这些视图，但是当我在模拟器或实际设备上运行应用程序时，文本似乎丢失了.为了更加清晰，我将在下面附加照片.

I've added a text and image views and on the Preview section they are shown as they should be but when I run the app on the emulator or on an actual device the text seem to be missing. I am going to attach photos below for more clarity.

自定义颜色是材料蓝色500"(#2196F3).尝试将其更改为另一个，但问题似乎并未受到影响.文字大小为40sp(在600x600的设备上为80sp).

The custom colour is Material Blue 500 (#2196F3). Tried changing it to another but the problem doesn't seem to be affected by it. The text size is 40sp (80sp on a > 600x600 device).