avoid possible NPEs when extracting dependencies

in general, we are keeping the code here in sync with code that
1) we expect to end up in sbt eventually too
2) contains fixes that arose during testing of the incremental compiler
   on at least one of the same codebases we expect scala-sculpt to be
   used with

the change to symbolsInType is borrowed from
  adriaanm/sbt#1,
specifically from
  adriaanm/sbt@98e9cff
the sbt test case that triggers it is
  https://github.com/sbt/sbt/tree/0.13/sbt/src/sbt-test/source-dependencies/macro-annotation
but we cannot add a similar test case here unless we expanded our test
scaffold a lot, since we'd need to add a dependency on macro paradise
and run the compiler twice (once for the macro, once for rest of the test code)

the change to flattenTypeToSymbols is borrowed from
  adriaanm/sbt@dc60968

Author: SethTisue

src/main/scala/scala/tools/sculpt/plugin/ExtractDependencies.scala

@@ -95,8 +95,8 @@ abstract class ExtractDependencies extends PluginComponent {
     }
 
     // skip packages
-    private def symbolsInType(tp: Type) = tp.collect{ case tp if !(tp.typeSymbol hasFlag PACKAGE) => tp.typeSymbolDirect }.toSet
-    private def flattenTypeToSymbols(tp: Type): List[Symbol] = tp match {
+    private def symbolsInType(tp: Type) = tp.collect{ case tp if tp != null && !(tp.typeSymbolDirect hasFlag PACKAGE) => tp.typeSymbolDirect }.toSet
+    private def flattenTypeToSymbols(tp: Type): List[Symbol] = if (tp eq null) Nil else tp match {
       case ct: CompoundType => ct.typeSymbolDirect :: ct.parents.flatMap(flattenTypeToSymbols)
       case _ => List(tp.typeSymbolDirect)
     }