ClassCastException when dropping a STREAM with 2+ INSERT INTO statements

[mikebin]

Describe the bug
The following exception occurs when attempting to drop a STREAM which is associated with 2+ INSERT INTO statements:

class io.confluent.ksql.query.QueryId cannot be cast to class java.lang.Comparable (io.confluent.ksql.query.QueryId is in unnamed module of loader 'app'; java.lang.Comparable is in module java.base of loader 'bootstrap')

To Reproduce
ksqlDB 0.20.0

(Contrived example to illustrate the bug)

create stream s (val string) with (kafka_topic='s', value_format='json', partitions=1);
insert into s select * from s;
insert into s select * from s;

drop stream s delete topic;

Expected behavior
User-friendly error message explaining why stream S cannot be dropped, such as:

Cannot drop S.
The following queries read from this source: [INSERTQUERY_2, INSERTQUERY_3].
The following queries write into this source: [INSERTQUERY_2, INSERTQUERY_3].
You need to terminate them before dropping S.

Actual behaviour
The following error is returned to the user in the CLI:

class io.confluent.ksql.query.QueryId cannot be cast to class java.lang.Comparable (io.confluent.ksql.query.QueryId is in unnamed module of loader 'app'; java.lang.Comparable is in module java.base of loader 'bootstrap')

ksqlDB Server log:

ksqldb-server       | io.confluent.ksql.util.KsqlStatementException: class io.confluent.ksql.query.QueryId cannot be cast to class
 java.lang.Comparable (io.confluent.ksql.query.QueryId is in unnamed module of loader 'app'; java.lang.Comparable is in module jav
a.base of loader 'bootstrap')
ksqldb-server       | Statement: DROP STREAM S;
ksqldb-server       |   at io.confluent.ksql.engine.EngineExecutor.executeDdl(EngineExecutor.java:610)
ksqldb-server       |   at io.confluent.ksql.engine.EngineExecutor.lambda$execute$0(EngineExecutor.java:138)
ksqldb-server       |   at java.base/java.util.Optional.map(Optional.java:265)
ksqldb-server       |   at io.confluent.ksql.engine.EngineExecutor.execute(EngineExecutor.java:138)
ksqldb-server       |   at io.confluent.ksql.engine.SandboxedExecutionContext.execute(SandboxedExecutionContext.java:139)
ksqldb-server       |   at io.confluent.ksql.rest.server.computation.ValidatedCommandFactory.createForPlannedQuery(ValidatedComman
dFactory.java:139)
ksqldb-server       |   at io.confluent.ksql.rest.server.computation.ValidatedCommandFactory.createCommand(ValidatedCommandFactory
.java:106)
ksqldb-server       |   at io.confluent.ksql.rest.server.computation.ValidatedCommandFactory.create(ValidatedCommandFactory.java:6
6)
ksqldb-server       |   at io.confluent.ksql.rest.server.validation.RequestValidator.validate(RequestValidator.java:164)
ksqldb-server       |   at io.confluent.ksql.rest.server.validation.RequestValidator.validate(RequestValidator.java:130)
ksqldb-server       |   at io.confluent.ksql.rest.server.resources.KsqlResource.handleKsqlStatements(KsqlResource.java:281)
ksqldb-server       |   at io.confluent.ksql.rest.server.KsqlServerEndpoints.lambda$executeKsqlRequest$2(KsqlServerEndpoints.java:
202)
ksqldb-server       |   at io.confluent.ksql.rest.server.KsqlServerEndpoints.lambda$executeOldApiEndpointOnWorker$23(KsqlServerEnd
points.java:356)
ksqldb-server       |   at io.confluent.ksql.rest.server.KsqlServerEndpoints.lambda$executeOnWorker$22(KsqlServerEndpoints.java:34
2)
ksqldb-server       |   at io.vertx.core.impl.ContextImpl.lambda$executeBlocking$2(ContextImpl.java:313)
ksqldb-server       |   at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
ksqldb-server       |   at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
ksqldb-server       |   at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
ksqldb-server       |   at java.base/java.lang.Thread.run(Thread.java:829)
ksqldb-server       | Caused by: java.lang.ClassCastException: class io.confluent.ksql.query.QueryId cannot be cast to class java.
lang.Comparable (io.confluent.ksql.query.QueryId is in unnamed module of loader 'app'; java.lang.Comparable is in module java.base
 of loader 'bootstrap')
ksqldb-server       |   at java.base/java.util.Comparators$NaturalOrderComparator.compare(Comparators.java:47)
ksqldb-server       |   at java.base/java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
ksqldb-server       |   at java.base/java.util.TimSort.sort(TimSort.java:220)
ksqldb-server       |   at java.base/java.util.Arrays.sort(Arrays.java:1515)
ksqldb-server       |   at java.base/java.util.stream.SortedOps$SizedRefSortingSink.end(SortedOps.java:353)
ksqldb-server       |   at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:485)
ksqldb-server       |   at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
ksqldb-server       |   at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
ksqldb-server       |   at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
ksqldb-server       |   at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
ksqldb-server       |   at io.confluent.ksql.engine.EngineContext.throwIfInsertQueriesExist(EngineContext.java:261)
ksqldb-server       |   at io.confluent.ksql.engine.EngineContext.executeDdl(EngineContext.java:223)
ksqldb-server       |   at io.confluent.ksql.engine.EngineExecutor.executeDdl(EngineExecutor.java:606)
ksqldb-server       |   ... 18 more

Additional context
This code attempts to sort a set of QueryId, but QueryId is not a Comparable: https://github.com/confluentinc/ksql/blob/0.20.0-ksqldb/ksqldb-engine/src/main/java/io/confluent/ksql/engine/EngineContext.java#L259. Note: sorted is a no-op if there's only one QueryId in the set, so this bug only occurs if there are 2+ INSERT INTO statements read from/writing to the stream.