Wpf porpertygrid issue with collection inside a collection

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WPF

PropertyGrid

I have a WPF application which has a property grid. I am binding the property grid with a class object (say the class name is "mainclass"). The "mainclass" has a property "APIConditions" which is an Observable collection of another custom class "SearchAPIModel". "SearchAPIModel" has a property of observablecollection of class "KeyValues". I have added an item to the "APIConditions" collection using the UI of property grid. The newly added object to the collection is saving successfully. But the problem I am facing is, I am able to bind the "main class" object to the property grid but when I am trying to open the "APIConditions" property which is supposed to open in a popupwindow, the application crashes with the error
"An unhandled exception of type 'System.Reflection.TargetParameterCountException' occurred in PresentationFramework.dll
Additional information: Parameter count mismatch."

Can anyone explain why this issue occurs and what is the solution for this ?.

Thanks in advance,
Rasheed

What I have tried:

Please see the classes as below.

[Serializable]
public class MainClass : INotifyPropertyChanged, INotifyCollectionChanged
{
    private string _SelectText, _ID;      
    private ObservableCollection<searchapimodel> _APIConditions;       
    public event PropertyChangedEventHandler PropertyChanged;
    public event NotifyCollectionChangedEventHandler CollectionChanged;
   
    private void OnNotifyPropertyChanged(string propertyName)
    {
        var tmp = PropertyChanged;
        if (tmp != null)
        {
            tmp(this, new PropertyChangedEventArgs(propertyName));
        }
    }      
  
    [CategoryAttribute("TextBox Details"), DefaultValueAttribute(true), DescriptionAttribute("ID of the TextBox control"), ReadOnly(true)]
    public string ID
    {
        get { return _ID; }
        set
        {

            if (_ID != value)
            {
                _ID = value;
            }
        }
    }

    [CategoryAttribute("LookupOption Details"), DefaultValueAttribute(true), DescriptionAttribute("Initial Text For Lookup And Nothing is Selected"), ReadOnly(false)]
    public string SelectText
    {
        get { return _SelectText; }
        set
        {
            if (_SelectText != value)
            {
                _SelectText = value;                   
            }
        }
    }

    [CategoryAttribute("LookupOption Details"), DefaultValueAttribute(true), DescriptionAttribute("Collection Of SearchAPIModel.")]
    public ObservableCollection<searchapimodel> APIConditions
    {
        get { return _APIConditions; }
        set
        {
            if (_APIConditions != value)
            {
                _APIConditions = value;
            }
        }
    }
    public MainClass()
    {
        this.SelectText = "[SELECT]";
        this.ID = "";
        this.APIConditions = new ObservableCollection<searchapimodel>();
    }
}

SearchAPIModel class

[Serializable]
public class SearchAPIModel : INotifyPropertyChanged, INotifyCollectionChanged
{

    private string _ConditionRow;
    public event PropertyChangedEventHandler PropertyChanged;
    public event NotifyCollectionChangedEventHandler CollectionChanged;
    
    private ObservableCollection<keyvalues> _SearchTextList;        
    
    private void OnNotifyPropertyChanged(string propertyName)
    {
        var tmp = PropertyChanged;
        if (tmp != null)
        {
            tmp(this, new PropertyChangedEventArgs(propertyName));
        }
    }
    [CategoryAttribute("API Condition Details"), DefaultValueAttribute(true), DescriptionAttribute("ConditionRow")]
    public string ConditionRow
    {
        get { return _ConditionRow; }
        set
        {
            if (_ConditionRow != value)
            {
                _ConditionRow = value;
                OnNotifyPropertyChanged("");
            }
        }
    }

    [CategoryAttribute("API Condition Details"), DefaultValueAttribute(true), DescriptionAttribute("SearchTextList")]
    public ObservableCollection<keyvalues> SearchTextList
    {
        get { return _SearchTextList; }
        set
        {
            _SearchTextList = value;
            if (_SearchTextList != value)
            {
                _SearchTextList = value;
            }
        }
    }

    public SearchAPIModel()
    {
        this.ConditionRow = "";
        this.SearchTextList = new ObservableCollection<keyvalues>();            
    }
  
}

KeyValues class

[Serializable]
public class KeyValues : INotifyPropertyChanged
{
    private string _key, _Value;
    [ReadOnly(false)]
    public string Key
    {
      get { return _key; }
      set { _key = value;  }
    }
    [ReadOnly(false)]
    public string Value
    {
        get { return _Value; }
        set { _Value = value; }
    }

    public KeyValues()
    {
        Key = "";
        Value = "";
    }

    public event PropertyChangedEventHandler PropertyChanged;
    private void OnNotifyPropertyChanged()
    {
        var tmp = PropertyChanged;
        if (tmp != null)
        {
            tmp(this, new PropertyChangedEventArgs(""));
        }
    }
}