It is bit difficult to say what is going wrong from given stack trace.
It would be good if you can attach complete log file.
(Is ALTER running as RSU or TOI ?)
Also, it seems like you are using quite-old version of PXC. Latest one is 5.6.30 and in
meantime quite some important bugs has been fixed.
You may want to try upgrade.
It is bit difficult to say what is going wrong from given stack trace.
It would be good if you can attach complete log file.
(Is ALTER running as RSU or TOI ?)
Also, it seems like you are using quite-old version of PXC. Latest one is 5.6.30 and in
meantime quite some important bugs has been fixed.
You may want to try upgrade.