tmf: Bug 582385: Fix StackOverflowException in EventDensity view

The this patch fixes a StackOverflowException in the
HistogramDataProviderFactory when a TmfTrace extension filters out the
framework, default TmfStatisticsModule. The factory now returns null
instead of falsely calling the TmfTreeXYCompositeDataProvider which
ends-up in an endless recursion.

Even though it's a default analysis module, extensions can choose to
remove it and the framework view should not cause such an exception.

[Fixed] Bug 582385: Fix StackOverflowException in EventDensity view

Change-Id: Id904867f7559bcd6cc93588fffabe47c7a5a1c77
Signed-off-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/c/tracecompass/org.eclipse.tracecompass/+/204200
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Trace Compass Bot <tracecompass-bot@eclipse.org>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>

tmf/org.eclipse.tracecompass.tmf.core/src/org/eclipse/tracecompass/internal/tmf/core/histogram/HistogramDataProviderFactory.java

@@ -55,6 +55,7 @@ public class HistogramDataProviderFactory implements IDataProviderFactory {
                 statsMod.schedule();
                 return new HistogramDataProvider(trace, statsMod);
             }
+            return null;
         }
         return TmfTreeXYCompositeDataProvider.create(traces, HistogramDataProvider.TITLE, HistogramDataProvider.ID);
     }