Added preconditions to AcmDevice to check for null pointers

android_acm_serial/src/org/ros/android/acm_serial/AcmDevice.java

@@ -42,6 +42,8 @@ public class AcmDevice {
   private final OutputStream outputStream;
 
   public AcmDevice(UsbDeviceConnection usbDeviceConnection, UsbInterface usbInterface) {
+    Preconditions.checkNotNull(usbDeviceConnection);
+    Preconditions.checkNotNull(usbInterface);  
     Preconditions.checkState(usbDeviceConnection.claimInterface(usbInterface, true));
     this.usbDeviceConnection = usbDeviceConnection;