[RDF] prevent nullptr access in HasColumn when no datasource defined (#19835)

* [RDF] prevent nullptr access in HasColumn when no datasource defined

Fixes #19834


Co-authored-by: Philippe Canal <[email protected]>

Author: ferdymercury

tree/dataframe/src/RInterfaceBase.cxx

@@ -274,7 +274,7 @@ bool ROOT::RDF::RInterfaceBase::HasColumn(std::string_view columnName)
    if (fColRegister.IsDefineOrAlias(columnName))
       return true;
 
-   if (auto ds = GetDataSource(); ds->HasColumn(columnName))
+   if (auto ds = GetDataSource(); ds && ds->HasColumn(columnName))
       return true;
 
    return false;

tree/dataframe/test/dataframe_colnames.cxx

@@ -45,6 +45,15 @@ TEST(ColNames, ContainedNames)
    EXPECT_EQ(1U, *c);
 }
 
+// Test for https://github.com/root-project/root/issues/19834
+TEST(ColNames, NoSource)
+{
+   auto df = ROOT::RDataFrame(1);
+   auto df_def = df.Define("x", "int(1)");
+   EXPECT_TRUE(df_def.HasColumn("x"));
+   EXPECT_FALSE(df_def.HasColumn("y"));
+}
+
 TEST(Aliases, DefineOnAlias)
 {
    ROOT::RDataFrame tdf(2);