on adding one button to the webpart this error is coming what is this error?

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SharePoint2013

Server Error in '/' Application.
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Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request.

Source Error:


An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:



[HttpException (0x80004005): Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request.]
System.Web.UI.Control.LoadViewStateRecursive(Object savedState) +12659837
System.Web.UI.Control.LoadChildViewStateByIndex(ArrayList childState) +216
System.Web.UI.Control.LoadViewStateRecursive(Object savedState) +381
System.Web.UI.Control.LoadChildViewStateByID(ArrayList childState) +177
System.Web.UI.Control.LoadViewStateRecursive(Object savedState) +368
System.Web.UI.Control.LoadChildViewStateByIndex(ArrayList childState) +216
System.Web.UI.Control.LoadViewStateRecursive(Object savedState) +381
System.Web.UI.Control.LoadChildViewStateByIndex(ArrayList childState) +216
System.Web.UI.Control.LoadViewStateRecursive(Object savedState) +381
System.Web.UI.Control.LoadChildViewStateByIndex(ArrayList childState) +216
System.Web.UI.Control.LoadViewStateRecursive(Object savedState) +381
System.Web.UI.Control.LoadChildViewStateByIndex(ArrayList childState) +216
System.Web.UI.Control.LoadViewStateRecursive(Object savedState) +381
System.Web.UI.Control.LoadChildViewStateByIndex(ArrayList childState) +216
System.Web.UI.Control.LoadViewStateRecursive(Object savedState) +381
System.Web.UI.Page.LoadAllState() +563
System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2533




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Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.18010