Android ImageView NullPointerException

I have two images, a red light and a green light. I have a custom ListView that I would like to display a red light when a list item is inactive, and a green light when it is active. A list item is activated when it is pressed. Here is my code row.xml ``` <ImageView android:id="@+id/iconLight" android:src="@drawable/light_off" android:layout_width="wrap_content" android:layout_height="wrap_content"/> ``` main.java ``` ImageView iconLight = (ImageView)findViewById(R.id.iconLight); iconLight.setImageResource(R.drawable.light_on); ``` I get a NullPointerException executing the line that sets the image resource. So I did a little testing, I deleted the line setting the src in the XML file and just tried to set it in the main class. Still a NPE. So I tried not changing the resource, but just changing the alpha. Still NPE. I'm not sure what I'm doing wrong. The files `light_off.png` and `light_on.png` are both in `res/drawable-ldpi` and either of them work when I specify them in the XML. But any change I attempt to make to `iconLight` in the main file causes this NPE. Any ideas?