Steps to reproduce 

In order to quickly reproduce the issue we are going to force assets to have subnodes

1. Go to JCR app and navigate to /tours/shark_brian_warrick_0824.JPG and import the attached XML dam.tours.shark_brian_warrick_0824..JPG.test.xml
2. Add restorePreviousVersion to the action bar in dam assets app: edit /dam-app-jcr/decorations/dam-assets-app/apps/dam.subApps.yaml to add it as follows: 

   versions:
               items:
                 - name: showVersions
                 - name: restoreVersion
                 - name: restorePreviousVersion
   

3. Open assets app and go to /tours/shark_brian_warrick_0824.JPG. Publish it to make sure there are versions available
4. Click on restore previous action in the action bar

Expected results

Previous version is restored ok

Actual results

An error is thrown with no detailed information of the cause

Workaround

Using restore version action

Development notes

This error happens only when there are subnodes in the item. When debugging RestorePreviousVersionCommand you can see that the cause is a nullPointerException which is thrown in info.magnolia.commands.impl.RestorePreviousVersionCommand.getPreviousVersion(Node) method, specifically here:

VersionIterator versionIterator = versionManager.getAllVersions(node);

        while (versionIterator.hasNext()) {
            previousVersion = versionIterator.nextVersion();
        }

versionManager.getAllVersions can return null in two scenarios, if versionedNode is null and if versionHistory is null. Therefore, if one of these situations occurs, the next instruction, versionIterator.hasNext(), will throw a null pointer exception.
SOLUTION: add a null control before the while loop:

 VersionIterator versionIterator = versionManager.getAllVersions(node);
if(versionIterator!=null){
        while (versionIterator.hasNext()) {
            previousVersion = versionIterator.nextVersion();
        }
}