Why deactivating storage domain is failing with error "java.lang.NullPointerException"

Issue

• The storage domain fails to deactivate and is showing below error in the engine log.

[org.ovirt.engine.core.bll.storage.ovfstore.ProcessOvfUpdateForStoragePoolCommand] (default task-1209) [7889a20d] Exception: java.lang.NullPointerException
        at java.util.HashMap.merge(HashMap.java:1225) [rt.jar:1.8.0_222]
        at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320) [rt.jar:1.8.0_222]
        at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169) [rt.jar:1.8.0_222]
        at java.util.HashMap$KeySpliterator.forEachRemaining(HashMap.java:1556) [rt.jar:1.8.0_222]
        at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482) [rt.jar:1.8.0_222]
        at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472) [rt.jar:1.8.0_222]
        at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) [rt.jar:1.8.0_222]
        at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) [rt.jar:1.8.0_222]
        at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:566) [rt.jar:1.8.0_222]
        at org.ovirt.engine.core.utils.ovf.OvfManager.getMemoryDiskForSnapshots(OvfManager.java:81) [bll.jar:]