Disallow null callbacks.

Bug: 31815793
Test: hidl_test
Change-Id: I3a977df3cb36af46fa5343c1ebbe97df4ff23fca
diff --git a/AST.h b/AST.h
index 46d3c0b..88f2a43 100644
--- a/AST.h
+++ b/AST.h
@@ -140,6 +140,8 @@
     std::string makeHeaderGuard(const std::string &baseName) const;
     void enterLeaveNamespace(Formatter &out, bool enter) const;
 
+    static void generateCheckNonNull(Formatter &out, const std::string &nonNull);
+
     status_t generateInterfaceHeader(const std::string &outputPath) const;
     status_t generateHwBinderHeader(const std::string &outputPath) const;
     status_t generateStubHeader(const std::string &outputPath) const;