content_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:weightSum="1"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="com.example.todos.TodoListActivity"
    tools:showIn="@layout/activity_main">

        <Spinner
            android:id="@+id/spinCategories"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:entries="@array/categories_array"     
             />
        <ListView
            android:id="@+id/lvTodos"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_below="@id/spinCategories"
            android:layout_marginTop="15dp"
            />
</RelativeLayout>

It would help if you upload you "dimens.xml" file because on Android 3.0+ it is giving an error with:

android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"

Fixed it by, going to res > values > dimens.xml and added this to resources:

<dimen name="activity_horizontal_margin">16dp</dimen>
<dimen name="activity_vertical_margin">16dp</dimen>