Hello,

StringBuilder content = new StringBuilder(); yields
hudson.remoting.ProxyException: org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object '' with class 'java.lang.String' to class 'java.lang.StringBuilder'
error.

Full backtrace:

 
hudson.remoting.ProxyException: org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object '' with class 'java.lang.String' to class 'java.lang.StringBuilder'
at org.codehaus.groovy.runtime.typehandling.DefaultTypeTransformation.continueCastOnSAM(DefaultTypeTransformation.java:405)
at org.codehaus.groovy.runtime.typehandling.DefaultTypeTransformation.continueCastOnNumber(DefaultTypeTransformation.java:319)
at org.codehaus.groovy.runtime.typehandling.DefaultTypeTransformation.castToType(DefaultTypeTransformation.java:232)
at org.codehaus.groovy.runtime.DefaultGroovyMethods.asType(DefaultGroovyMethods.java:15537)
at org.codehaus.groovy.runtime.StringGroovyMethods.asType(StringGroovyMethods.java:187)
at org.codehaus.groovy.runtime.StringGroovyMethods.asType(StringGroovyMethods.java:110)
at org.codehaus.groovy.runtime.dgm$1046.doMethodInvoke(Unknown Source)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1218)
at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1027)
at org.codehaus.groovy.runtime.InvokerHelper.invokePojoMethod(InvokerHelper.java:913)
at org.codehaus.groovy.runtime.InvokerHelper.invokeMethod(InvokerHelper.java:904)
at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.invokeMethodN(ScriptBytecodeAdapter.java:168)
at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.asType(ScriptBytecodeAdapter.java:591)
at org.kohsuke.groovy.sandbox.impl.Checker.checkedCast(Checker.java:584)
at org.kohsuke.groovy.sandbox.impl.Checker$checkedCast$0.callStatic(Unknown Source)
at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCallStatic(CallSiteArray.java:56)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callStatic(AbstractCallSite.java:194)
at org.kohsuke.groovy.sandbox.impl.Checker$2.call(Checker.java:186)
at org.kohsuke.groovy.sandbox.GroovyInterceptor.onStaticCall(GroovyInterceptor.java:35)
at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onStaticCall(SandboxInterceptor.java:158)
at org.kohsuke.groovy.sandbox.impl.Checker$2.call(Checker.java:184)
at org.kohsuke.groovy.sandbox.impl.Checker.checkedStaticCall(Checker.java:188)
at org.kohsuke.groovy.sandbox.impl.Checker.checkedCall(Checker.java:95)
at com.cloudbees.groovy.cps.sandbox.SandboxInvoker.methodCall(SandboxInvoker.java:17)
at WorkflowScript.notifyFlowdockDeployStatus(WorkflowScript:223)
at WorkflowScript.run(WorkflowScript:140)
at __cps.transform__(Native Method)
at com.cloudbees.groovy.cps.impl.ContinuationGroup.methodCall(ContinuationGroup.java:57)
at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.dispatchOrArg(FunctionCallBlock.java:109)
at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.fixArg(FunctionCallBlock.java:82)
at sun.reflect.GeneratedMethodAccessor380.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72)
at com.cloudbees.groovy.cps.impl.ConstantBlock.eval(ConstantBlock.java:21)
at com.cloudbees.groovy.cps.Next.step(Next.java:83)
at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:174)
at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:163)
at org.codehaus.groovy.runtime.GroovyCategorySupport$ThreadCategoryInfo.use(GroovyCategorySupport.java:122)
at org.codehaus.groovy.runtime.GroovyCategorySupport.use(GroovyCategorySupport.java:261)
at com.cloudbees.groovy.cps.Continuable.run0(Continuable.java:163)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.access$001(SandboxContinuable.java:19)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable$1.call(SandboxContinuable.java:35)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable$1.call(SandboxContinuable.java:32)
at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.GroovySandbox.runInSandbox(GroovySandbox.java:108)
at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.run0(SandboxContinuable.java:32)
at org.jenkinsci.plugins.workflow.cps.CpsThread.runNextChunk(CpsThread.java:174)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.run(CpsThreadGroup.java:330)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.access$100(CpsThreadGroup.java:82)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:242)
at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:230)
at org.jenkinsci.plugins.workflow.cps.CpsVmExecutorService$2.call(CpsVmExecutorService.java:64)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at hudson.remoting.SingleLaneExecutorService$1.run(SingleLaneExecutorService.java:112)
at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)
 

code used https://github.com/jenkinsci/flowdock-plugin/issues/24#issuecomment-271784565