Throw on null terms in term trie (relates to MPSCR-40)

For more clear errors stemming from usage of constraints with null values, which ain't handled

reactor/Core/src/jetbrains/mps/logic/reactor/util/ClassicTermTrie.kt

@@ -147,7 +147,7 @@ class ClassicTermTrie<T> : TermTrie<T> {
             WILDCARD
 
         } else {
-            term.symbol()
+            term.symbol() ?: throw NullPointerException("term symbol can't be null")
         }
     }