Add null check.

h2/src/main/org/h2/command/ddl/AlterTableAddConstraint.java

@@ -198,6 +198,9 @@ public class AlterTableAddConstraint extends SchemaCommand {
         }
         case CommandInterface.ALTER_TABLE_ADD_CONSTRAINT_REFERENTIAL: {
             Table refTable = refSchema.resolveTableOrView(session, refTableName);
+            if (refTable == null) {
+                throw DbException.get(ErrorCode.TABLE_OR_VIEW_NOT_FOUND_1, refTableName);
+            }
             session.getUser().checkRight(refTable, Right.ALL);
             if (!refTable.canReference()) {
                 throw DbException.getUnsupportedException("Reference " +