Commented Unassigned: Null reference exception unhandled [166]

i have return a code and run it successfully but when ever i plot more than three graph plotting function it shows null reference exception , even I try to run a plotting function once again it also shows null reference exception error i don't know what i have done wrong please help me its a urgent matter for me my code is below and i will number the areas where the null reference exception occurring

i will send u the full project code as text file. Help me to solve the problem
Comments: this is the stack trace of null reference exception ``` at RDotNet.Internals.DirectNativeMethods._R_tryEval(IntPtr statement, IntPtr environment, Boolean& errorOccurred) at RDotNet.Internals.DirectNativeMethods.R_tryEval(IntPtr statement, IntPtr environment, Boolean& errorOccurred) at RDotNet.Expression.TryEvaluate(Environment environment, SymbolicExpression& result) at RDotNet.REngine.Parse(String statement, StringBuilder incompleteStatement) at RDotNet.REngine.<Evaluate>d__0.MoveNext() at System.Linq.Enumerable.LastOrDefault[TSource](IEnumerable`1 source) at RDotNet.REngine.EagerEvaluate(String statement) at WindowsFormApllication8.Form2.button1_Click(Object sender, EventArgs e) in C:\Users\nilkhil rajendran\Documents\WindowsFormsApplication8\WindowsFormsApplication8\Form2.cs:line 321 at System.Windows.Forms.Control.OnClick(EventArgs e) at System.Windows.Forms.Button.OnClick(EventArgs e) at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent) at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) at System.Windows.Forms.Control.WndProc(Message& m) at System.Windows.Forms.ButtonBase.WndProc(Message& m) at System.Windows.Forms.Button.WndProc(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg) at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData) at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context) at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context) at System.Windows.Forms.Application.Run(Form mainForm) at WindowsFormsApplication8.Program.Main() in C:\Users\nilkhil rajendran\Documents\WindowsFormsApplication8\WindowsFormsApplication8\Program.cs:line 18 at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args) at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args) at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() at System.Threading.ThreadHelper.ThreadStart_Context(Object state) at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart() ``` did you understand anything that cause exception from here I am not more familiar with c#