Fix a NPE in the presentation compiler

We were using a TypingTransformer and we called
`atOwner` before we had called `transform`. This
meant that `currTree` was null, which was observed
when that was passed to `Context#make`.

IDE ticket:

    https://scala-ide-portfolio.assembla.com/spaces/scala-ide/tickets/1001971#/activity/ticket:

Stack trace:

    exception during macro expansion: java.lang.NullPointerException
    at scala.tools.nsc.interactive.ContextTrees$class.addContext(ContextTrees.scala:78)
    at scala.tools.nsc.interactive.Global.addContext(Global.scala:28)
    at scala.tools.nsc.interactive.Global.registerContext(Global.scala:268)
    at scala.tools.nsc.typechecker.Contexts$Context.make(Contexts.scala:295)
    at scala.tools.nsc.typechecker.Contexts$Context.make0(Contexts.scala:320)
    at scala.tools.nsc.typechecker.Contexts$Context.make(Contexts.scala:327)
    at scala.tools.nsc.typechecker.Typers$Typer.atOwner(Typers.scala:5662)
    at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:33)
    at scala.tools.nsc.transform.TypingTransformers$TypingTransformer.atOwner(TypingTransformers.scala:28)
    at scala.async.internal.AsyncTransform$class.fixup$1(AsyncTransform.scala:191)