After upgrading to Sitecore 7.2 we got the following exception when trying to build our custom indexes:

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object.
 at Sitecore.ContentSearch.LuceneProvider.LuceneIndex.Reset()
 at Sitecore.ContentSearch.LuceneProvider.LuceneIndex.PerformRebuild(IndexingOptions indexingOptions, CancellationToken cancellationToken)
 at Sitecore.ContentSearch.LuceneProvider.LuceneIndex.Rebuild()
 --- End of inner exception stack trace ---
 at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
 at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
 at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
 at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
 at (Object , Object[] )
 at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
 at Sitecore.Jobs.Job.ThreadEntry(Object state)

The resolution to this is quite simple. Sitecore has included <initializeOnAdd> to the index configuration. If this field is missing in the configuration, building the index fails. Just make sure you have the following line to your custom index configuration:

<myCustomIndexConfiguration type="Sitecore.ContentSearch.LuceneProvider.LuceneIndexConfiguration, Sitecore.ContentSearch.LuceneProvider">

<indexAllFields>true</indexAllFields>

<!-- Needed since SC7.2 -->
<initializeOnAdd>true</initializeOnAdd>
 ....