App results in blank black screen on android phone....why?

I have removed all the elements from my layout and commented out references to the controls etc that I did have in my layout:

Code:

---

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:background="#0099cc"
    tools:context=".HexapodControls"
    android:id="@+id/frame_controls"
    android:visibility="visible"
    android:paddingTop="50dp">

    <!-- The primary full-screen view. This can be replaced with whatever view
        is needed to present your content, e.g. VideoView, SurfaceView,
        TextureView, etc. -->

    <!-- This FrameLayout insets its children based on system windows using
        android:fitsSystemWindows. -->

</FrameLayout>

---

So why, when I generate the apk in android studio and install it on my phone, do I get a blank black screen.

I give up! What have I changed to cause this?