DataIndex addition on SourceTable coalesce breaks if index columns are dropped via view

[rcaudy]

heduler-Concurrent-1 | i.d.s.s.SessionState      | Internal Error '41587ce6-da87-4c6f-8d75-33d2d33ca1fc' io.deephaven.engine.table.impl.locations.TableDataException: Error initializing location sizes
	at io.deephaven.engine.table.impl.SourceTable.lambda$initializeLocationSizes$3(SourceTable.java:197)
	at io.deephaven.engine.table.impl.perf.QueryPerformanceRecorder.withNugget(QueryPerformanceRecorder.java:353)
	at io.deephaven.engine.table.impl.SourceTable.initializeLocationSizes(SourceTable.java:191)
	at io.deephaven.engine.table.impl.SourceTable.initialize(SourceTable.java:117)
	at io.deephaven.engine.table.impl.SourceTable.doCoalesce(SourceTable.java:267)
	at io.deephaven.engine.table.impl.SourceTable.doCoalesce(SourceTable.java:33)
	at io.deephaven.engine.table.impl.UncoalescedTable.coalesce(UncoalescedTable.java:78)
	at io.deephaven.engine.table.impl.UncoalescedTable.flatten(UncoalescedTable.java:447)
	at io.deephaven.server.table.ops.FlattenTableGrpcImpl.create(FlattenTableGrpcImpl.java:29)
	at io.deephaven.server.table.ops.FlattenTableGrpcImpl.create(FlattenTableGrpcImpl.java:16)
	at io.deephaven.server.table.ops.TableServiceGrpcImpl$BatchExportBuilder.doExport(TableServiceGrpcImpl.java:757)
	at io.deephaven.server.session.SessionState$ExportObject.doExport(SessionState.java:995)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at io.deephaven.server.runner.scheduler.SchedulerModule$ThreadFactory.lambda$newThread$0(SchedulerModule.java:100)
	at java.base/java.lang.Thread.run(Thread.java:840)
Caused by: java.lang.NullPointerException
	at java.base/java.util.Objects.requireNonNull(Objects.java:209)
	at java.base/java.util.ImmutableCollections$MapN.get(ImmutableCollections.java:1232)
	at io.deephaven.engine.table.impl.indexer.DataIndexer$DataIndexCache.traverse(DataIndexer.java:443)
	at io.deephaven.engine.table.impl.indexer.DataIndexer$DataIndexCache.add(DataIndexer.java:507)
	at io.deephaven.engine.table.impl.indexer.DataIndexer.addDataIndex(DataIndexer.java:304)
	at io.deephaven.engine.table.impl.sources.regioned.RegionedColumnSourceManager.initialize(RegionedColumnSourceManager.java:251)
	at io.deephaven.engine.table.impl.SourceTable.lambda$initializeLocationSizes$3(SourceTable.java:195)
	... 18 more

We need to refuse to create indexes if we cannot find the appropriate column sources.