fix.

Author: trivialfis

src/gbm/gbtree.cc

@@ -636,7 +636,7 @@ GBTree::GetPredictor(HostDeviceVector<float> const *out_pred,
 
   // Data comes from SparsePageDMatrix. Since we are loading data in pages, no need to
   // prevent data copy.
-  if (!f_dmat->SingleColBlock()) {
+  if (f_dmat && !f_dmat->SingleColBlock()) {
     if (ctx_->IsCPU()) {
       return cpu_predictor_;
     } else {