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UI Automation is a feature that enables you to automate the interaction with UI elements on remote machines using native selectors. To use UI Automation in a scenario where multiple RDP connections are involved, you need to install the UiPath Windows Remote Desktop extension on the client machines and the UiPath Remote Runtime component on the destination machine. The extension allows you to identify UI elements on the remote desktop and send keyboard and mouse events to them. The Remote Runtime component enables the communication between the extension and the UiPath Robot service. In this case, Machine A and Machine B are client machines, while Machine C is the destination machine. Therefore, you need to install the RDP extension on Machine A and Machine B, and the Remote Runtime component on Machine C. (UiPath Studio documentation1)

1. Understanding the Error Message:

The error message shown in the Assign activity states:"Object reference not set to an instance of an object."

Exception Type: System.NullReferenceException

This error occurs when trying to use a variable that has not been initialized (i.e., it is Nothing or null in UiPath).

2. Why is this happening?

The variable PinMapping is a Dictionary (Dictionary<String, String>), but it has not been initialized before adding a key-value pair.

In UiPath, a dictionary must be initialized before assigning values.

Since PinMapping is Nothing (null), attempting to access or modify it causes a NullReferenceException.

3. Why Not Other Options?

"The 'John' key was not present in the dictionary." ❌

This would cause a KeyNotFoundException, not a NullReferenceException.

"The assign's set value syntax should be PinMapping<"John">." ❌

The syntax < > is incorrect for dictionary key access in UiPath; it uses brackets [ ].

"The assign's set value syntax should be PinMapping["John"]." ❌

The syntax is correct, but the actual issue is that the dictionary itself was not initialized.

4. Correcting the Issue:

Before using the dictionary, it must be initialized.

Solution 1: Initialize the Dictionary Before Use

In Variables Panel, set the default value of PinMapping as:

New Dictionary(Of String, String)

OR

Use an Assign activity before adding elements:

PinMapping = New Dictionary(Of String, String)

Solution 2: Check for Null Before Using

To avoid errors, check if PinMapping is Nothing:

If PinMapping Is Nothing Then

PinMapping = New Dictionary(Of String, String)

End If

Then, proceed with adding key-value pairs.

5. Reference from UiPath Official Documentation:

UiPath Dictionary Initialization

Handling NullReferenceException in UiPath

The best practice for storing credentials securely in UiPath is to use an Orchestrator Asset.

Orchestrator Assets allow secure storage and retrieval of sensitive information.

Credentials stored in Orchestrator cannot be exposed in logs and are encrypted.

When used in automation, the values are retrieved dynamically, reducing the risk of hardcoded credentials.

✅ Why other options are incorrect:

A (Hardcoded values): Storing credentials in the workflow is insecure and violates best practices.

B (Config.xlsx): Excel files are not secure and can be accessed by anyone with file permissions.

C (Local file): If credentials are stored locally, they become a security risk and are not easily maintainable across multiple bots.

✅ Reference: UiPath Orchestrator - Assets

Storing the username and password in a single Orchestrator asset is a recommended way to safely store credential information used in a UiPath automation project. This way, the credentials are encrypted and stored in a secure location, and can be accessed by the robot at runtime. Orchestrator assets also allow for easy management and updating of the credentials without modifying the workflow. References: Assets and Managing Assets in Studio from UiPath documentation.