fix: Ensure null is never put into visited types memory

Author: danez

src/utils/getFlowType.js

@@ -273,15 +273,15 @@ function getFlowTypeWithResolvedTypes(path: NodePath): FlowTypeDescriptor {
     }
   }
 
+  if (!type) {
+    type = { name: 'unknown' };
+  }
+
   if (isTypeAlias) {
     // mark the type as unvisited so that further calls can resolve the type again
     visitedTypes[path.parentPath.node.id.name] = type;
   }
 
-  if (!type) {
-    type = { name: 'unknown' };
-  }
-
   return type;
 }