Report error if schema null. (vesoft-inc#1700) (vesoft-inc#1711)

Co-authored-by: dutor <440396+dutor@users.noreply.github.com>
(cherry picked from commit 8a9daec)
(cherry picked from commit 65364c2)

Co-authored-by: CPWstatic <13495049+CPWstatic@users.noreply.github.com>

src/graph/FetchVerticesExecutor.cpp

@@ -343,6 +343,15 @@ void FetchVerticesExecutor::processAllPropsResult(RpcResponse &&result) {
                     return;
                 }
                 auto schema = ectx()->schemaManager()->getTagSchema(spaceId_, tdata.tag_id, ver);
+                if (schema == nullptr) {
+                    // It actually should never be null here.
+                    // But issue1699 indicates that it would be nullptr when schema
+                    // was altered. This is a hot fix through reporting error, and we will
+                    // find out why it is null.
+                    LOG(ERROR) << "Schema not found for id: " << tdata.tag_id;
+                    doError(Status::Error("Get schema failed when handle data."));
+                    return;
+                }
                 if (rsWriter == nullptr) {
                     outputSchema = std::make_shared<SchemaWriter>();
                     outputSchema->appendCol("VertexID", nebula::cpp2::SupportedType::VID);