Don't crash when taking the signature of NoType

I noted this happening from tyoime to time in the IDE, and it is super annoying
when it happens. Let's see if we can survive meaningfully after ignoring this condition.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -549,7 +549,7 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
         if (inst.exists) sigName(inst) else tpnme.Uninstantiated
       case tp: TypeProxy =>
         sigName(tp.underlying)
-      case _: ErrorType | WildcardType =>
+      case _: ErrorType | WildcardType | NoType =>
         tpnme.WILDCARD
       case tp: WildcardType =>
         sigName(tp.optBounds)