Fix crash when arcore not installed

app/src/main/java/com/googlecreativelab/drawar/DrawAR.java

@@ -388,6 +388,8 @@ public void onWindowFocusChanged(boolean hasFocus) {
      */
     @Override
     public void onSurfaceCreated(GL10 gl, EGLConfig config) {
+        if (mSession == null) return;
+
         GLES20.glClearColor(0.1f, 0.1f, 0.1f, 1.0f);
 
         // Create the texture and pass it to ARCore session to be filled during update().
@@ -527,7 +529,7 @@ public void onDrawFrame(GL10 gl) {
         mBackgroundRenderer.draw(mFrame);
 
         // Draw Lines
-        if (mFrame.getCamera().getTrackingState() == TrackingState.TRACKING) {
+        if (mFrame != null && mFrame.getCamera().getTrackingState() == TrackingState.TRACKING) {
             mLineShaderRenderer.draw(viewmtx, projmtx, mScreenWidth, mScreenHeight, AppSettings.getNearClip(), AppSettings.getFarClip());
         }
     }

app/src/main/java/com/googlecreativelab/drawar/rendering/BackgroundRenderer.java

@@ -129,6 +129,8 @@ public void createOnGlThread(Context context) {
      * @param frame The last {@code Frame} returned by {@link Session#update()}.
      */
     public void draw(Frame frame) {
+        if (frame == null) return;
+
         // If display rotation changed (also includes view size change), we need to re-query the uv
         // coordinates for the screen rect, as they may have changed as well.
         if (frame.hasDisplayGeometryChanged()) {