avoid premature crash due to missing session parents/imports;

--- a/src/Pure/Thy/sessions.scala	Fri Mar 20 19:55:42 2020 +0100
+++ b/src/Pure/Thy/sessions.scala	Fri Mar 20 20:23:40 2020 +0100
@@ -222,7 +222,7 @@
               val required_sessions =
                 dependencies.loaded_theories.all_preds(dependencies.theories.map(_.theory))
                   .map(theory => imports_base.theory_qualifier(theory))
-                  .filterNot(_ == info.name)
+                  .filter(name => name != info.name && sessions_structure.defined(name))
 
               val required_subgraph =
                 sessions_structure.imports_graph