Removing the last target from the graph data window in the composer causes a broken render.

[Dave Rawks]

When a new composer window is open there is no attempt made to render a graph before targets are defined in the graph data window. However once one or more target has been added, if you remove all of the targets the composer still attempts to render, but it generates a broken url like "http://localhost:8080/render/?width=586&height=308&_salt=1318278459.509&target="

It would be preferable if the composer did not attempt to render when there are no targets defined.

[Michael Leinartas (mleinartas)]

Fix committed to r693