Q:

XML not well formed (invalid token)

When I am trying to build my app using xamarin for android, an error pops up saying 

Error parsing XML: not well-formed (invalid token) in line 17

I cannot seem to find where the bad structure is though. As it seem, it has something to do with the way I try to include buttons in the grid layout. I am sure it is something stupid but I just cannot find it. I included my code below:
New error: 

Error parsing XML: unbound prefix in line 17

<?xml version="1.0" encoding="utf-8"?>

<GridLayout>
    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:minWidth="25px"
    android:minHeight="25px"
    android:layout_width="match_parent"
    android:layout_height="fill_parent"
    android:id="@+id/gridLayout1"
    android:paddingLeft="40"
    android:alignmentMode="alignMargins"
    android:columnCount="3"
    android:rowCount="20"
    <Button
        android:text="Button"
        android:id="@+id/button1"
        android:height="100"
        android:width="100" />
    <Button
        android:text="Button"
        android:id="@+id/button2"
        android:height="100"
        android:width="100" />
    <Button
        android:text="Button"
        android:id="@+id/button3"
        android:height="100"
        android:width="100" />
    <Button
        android:text="Button"
        android:id="@+id/button4"
        android:height="100"
        android:width="100" />
    <Button
        android:text="Button"
        android:id="@+id/button5"
        android:height="100"
        android:width="100" />
    <Button
        android:text="Button"
        android:id="@+id/button6"
        android:height="100"
        android:width="100" />
    <Button
        android:text="Button"
        android:id="@+id/button7"
        android:height="100"
        android:width="100" />
    <Button
        android:text="Button"
        android:id="@+id/button8"
        android:height="100"
        android:width="100" />
    <Button
        android:text="Button"
        android:id="@+id/button9"
        android:height="100"
        android:width="100" />
    <Button
        android:text="Button"
        android:id="@+id/button10"
        android:height="100"
        android:width="100" />
</GridLayout>

A:

If you count the line 17 that it points out to, you reach this line
android:height="100"

Notice the "unbound prefix" or how there is no unit (dp/sp) against that value.
Please fix that and all the other length measurements to contain a unit of measurement, so that the prefix is bound:
android:height="100dp"

