debugging

test/gtest_factor_gnss_single_diff_2D.cpp

@@ -122,6 +122,7 @@ class FactorGnssSingleDiff2DTest : public testing::Test
 TEST_F(FactorGnssSingleDiff2DTest, check_tree)
 {
     ASSERT_TRUE(problem_ptr->check(0));
+    ASSERT_TRUE(prior_frame_ptr != nullptr);
 }
 
 /*