Issue

• Individually ran SQL works

select <column1> from <table1>
select <column1> from <table2>

• following SQL also works

select null as <column1>
union
select <column1> from <table1>
union
select <column1> from <table2>

• UNION command throws an error ASSERTION FAILED: expected reference to be not null

select <column1> from <table1>
union
select <column1> from <table2>

• server.log entry similar to

ERROR \[org.teiid.PROCESSOR\] (Worker59_QueryProcessorQueue7410) TEIID30019 Unexpected exception for request BtFP6pSobsiE.14: java.lang.AssertionError: ASSERTION FAILED: expected reference to be not null
    at org.teiid.core.util.Assertion.failed(Assertion.java:73) [teiid-common-core-8.7.4.6_2-redhat-1.jar:8.7.4.6_2-redhat-1]
    at org.teiid.core.util.Assertion.isNotNull(Assertion.java:100) [teiid-common-core-8.7.4.6_2-redhat-1.jar:8.7.4.6_2-redhat-1]
    at org.teiid.core.util.Assertion.isNotNull(Assertion.java:92) [teiid-common-core-8.7.4.6_2-redhat-1.jar:8.7.4.6_2-redhat-1]
    at org.teiid.common.buffer.TupleBuffer.getBatch(TupleBuffer.java:286) [teiid-engine-8.7.4.6_2-redhat-1.jar:8.7.4.6_2-redhat-1]
    at org.teiid.dqp.internal.process.TupleSourceCache$CopyOnReadTupleSource.nextTuple(TupleSourceCache.java:103) [teiid-engine-8.7.4.6_2-redhat-1.jar:8.7.4.6_2-redhat-1]
    at org.teiid.query.processor.relational.AccessNode.nextBatchDirect(AccessNode.java:369) [teiid-engine-8.7.4.6_2-redhat-1.jar:8.7.4.6_2-redhat-1]...