Don’t crash with invalid values, such as malformed utf8.

See CocoaPods/CocoaPods#592.

ext/xcodeproj/xcodeproj_ext.c

@@ -103,7 +103,7 @@ hash_set(const void *keyRef, const void *valueRef, void *hash) {
 
       } else {
         CFStringRef descriptionRef = CFCopyDescription(elementRef);
-        // obviously not optimial, but we're raising here, so it doesn't really matter
+        // obviously not optimal, but we're raising here, so it doesn't really matter
         VALUE description = cfstr_to_str(descriptionRef);
         rb_raise(rb_eTypeError, "Plist array value contains a object type unsupported by Xcodeproj. In: `%s'", RSTRING_PTR(description));
         CFRelease(descriptionRef);
@@ -153,6 +153,10 @@ dictionary_set(st_data_t key, st_data_t value, CFMutableDictionaryRef dict) {
     valueRef = str_to_cfstr(value);
   }
 
+  if (valueRef == NULL) {
+    rb_raise(rb_eTypeError, "Unable to convert value of key `%s'.", RSTRING_PTR(rb_inspect(key)));
+  }
+
   CFDictionaryAddValue(dict, keyRef, valueRef);
   CFRelease(keyRef);
   CFRelease(valueRef);

spec/xcodeproj_ext_spec.rb

@@ -96,4 +96,10 @@ def o.to_hash; { 'from' => 'object' }; end
     end
     lambda { Xcodeproj.read_plist(@plist) }.should.raise TypeError
   end
+
+  it "it raises if for whatever reason the value could not be converted to a CFTypeRef" do
+    lambda do
+      Xcodeproj.write_plist({ "invalid" => "\xCA" }, @plist)
+    end.should.raise TypeError
+  end
 end