Ensure the plugin's native code is never called with an invalid NPP pointer.
diff --git a/samples/BrowserPlugin/jni/background/BackgroundPlugin.cpp b/samples/BrowserPlugin/jni/background/BackgroundPlugin.cpp
index 69e4854..f79d9ac 100644
--- a/samples/BrowserPlugin/jni/background/BackgroundPlugin.cpp
+++ b/samples/BrowserPlugin/jni/background/BackgroundPlugin.cpp
@@ -40,6 +40,7 @@
 extern ANPSurfaceInterfaceV0   gSurfaceI;
 extern ANPSystemInterfaceV0    gSystemI;
 extern ANPTypefaceInterfaceV0  gTypefaceI;
+extern ANPWindowInterfaceV0    gWindowI;
 
 #define ARRAY_COUNT(array)      (sizeof(array) / sizeof(array[0]))
 
@@ -53,6 +54,9 @@
 
 BackgroundPlugin::BackgroundPlugin(NPP inst) : SurfaceSubPlugin(inst) {
 
+    // initialize the java interface
+    m_javaInterface = NULL;
+
     // initialize the drawing surface
     m_surface = NULL;
 
@@ -71,6 +75,7 @@
 }
 
 BackgroundPlugin::~BackgroundPlugin() {
+    setJavaInterface(NULL);
     surfaceDestroyed();
 }