Ensure the plugin's native code is never called with an invalid NPP pointer.
diff --git a/samples/BrowserPlugin/jni/paint/PaintPlugin.cpp b/samples/BrowserPlugin/jni/paint/PaintPlugin.cpp
index 80f05e4..4222222 100644
--- a/samples/BrowserPlugin/jni/paint/PaintPlugin.cpp
+++ b/samples/BrowserPlugin/jni/paint/PaintPlugin.cpp
@@ -50,6 +50,9 @@
     memset(&m_colorToggle, 0, sizeof(m_colorToggle));
     memset(&m_clearSurface,  0, sizeof(m_clearSurface));
 
+    // initialize the java interface
+    m_javaInterface = NULL;
+
     // initialize the drawing surface
     m_surface = NULL;
 
@@ -85,6 +88,7 @@
     gPathI.deletePath(m_touchPath);
     gPaintI.deletePaint(m_paintSurface);
     gPaintI.deletePaint(m_paintButton);
+    setJavaInterface(NULL);
     surfaceDestroyed();
 }