apkpatch 抛异常，无法生成补丁文件

自己建了一个简单的helloword工程，结果用apkpatch.sh来使用，抛出如下异常：
java.lang.RuntimeException: can,t modified Field:VERSION_CODE(I), in class :Lmyself/myapplication/BuildConfig;
at com.euler.patch.diff.DiffInfo.addModifiedFields(DiffInfo.java:88)
at com.euler.patch.diff.DexDiffer.compareField(DexDiffer.java:125)
at com.euler.patch.diff.DexDiffer.compareField(DexDiffer.java:101)
at com.euler.patch.diff.DexDiffer.compareField(DexDiffer.java:95)
at com.euler.patch.diff.DexDiffer.diff(DexDiffer.java:32)
at com.euler.patch.ApkPatch.doPatch(ApkPatch.java:68)
at com.euler.patch.Main.main(Main.java:97)

Reproduce the failure with apkpatch.sh using the simple Hello World project, then start with DiffInfo.java:88 and follow the DexDiffer.compareField calls in the stack trace. Check how ApkPatch.doPatch reaches this error and determine what behavior is expected for BuildConfig.VERSION_CODE; done means the patch generation completes without this exception.

Written by the indexing model from the issue text.