fix: add null checks to prevent undefined access in callMethod

- Add null checks for methodToNumArgs and numArgsToNumRubyBlockArgs
- Fix 'Cannot read properties of undefined (reading '0')' error
- Ensure proper error handling for unknown function calls like 'abc'
- Restore proper 'wrong instruction' error messages instead of exceptions

This fixes the regression introduced by registerCallMyBlock implementation
where undefined function calls would cause exceptions instead of proper
error handling.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: takaokouji

src/lib/ruby-to-blocks-converter/index.js

@@ -395,7 +395,9 @@ class RubyToBlocksConverter {
         }
 
         const methodToNumArgs = this._receiverToMethods[receiverName];
+        if (!methodToNumArgs) return null;
         const numArgsToNumRubyBlockArgs = methodToNumArgs[name];
+        if (!numArgsToNumRubyBlockArgs) return null;
         const numRubyBlockArgsToCreateBlockFuncs = numArgsToNumRubyBlockArgs[args.length];
         if (!numRubyBlockArgsToCreateBlockFuncs) return null;