fixed null pointer error

Change-Id: I9d3e608c348cad5d5154b2827ffeb8696f9148ef
Generalized ColumnTreeNode to accept any ILinkableObject source.
Now comparing constructors in equals().
Fixed null pointer error.

Change-Id: I83b080270bf3bc8fcc285e97cf4b9ea76c7a91f4
WeaveTreeItem.createItems() now filters out nulls and no longer requires
that child items to be created as WeaveTreeItem objects.

Change-Id: Id6f429aa872411e993679036bf21e74025649616
fixed bug where data source wasn't being created if there was no target.

Change-Id: Id877a52f0cd006cfab0f252f9991d3d9593ea057
Simplified and improved DataSourceTreeNode using ColumnTreeNode.

Change-Id: Ib8f6e2ab3eb3efbee11d5bc37d63002ce2fb947e