ErrorCode 900 with the description "Exception in Capture: Object reference not set to an instance of an object" typically indicates a null reference exception. This error occurs when your code is attempting to use an object that has not been instantiated or has been set to `null`.

To troubleshoot and resolve this issue, you can follow these steps:

1. **Check Object Initialization**:
   Ensure that all objects you are using in your fingerprint capture code are properly instantiated before they are used. For example, if you have an object like `FingerprintDevice device`, make sure it is properly initialized:

   ```php
   $device = new FingerprintDevice();
   ```

2. **Null Checks**:
   Before using any object, add null checks to ensure that the object is not null. This can prevent the exception from occurring:

   ```php
   if ($device !== null) {
       $device->capture();
   } else {
       // Handle the case where the device is null
       echo "Device is not initialized.";
   }
   ```

3. **Review Capture Method**:
   If the exception occurs within the capture method itself, review the implementation of this method. Ensure that all dependencies and objects used within the method are properly initialized.

4. **Logs and Debugging**:
   Add logging to your code to track where the null reference might be occurring. This can help you pinpoint the exact location of the issue:

   ```php
   if ($device === null) {
       error_log("Device object is null.");
   } else {
       try {
           $device->capture();
       } catch (Exception $e) {
           error_log("Exception during capture: " . $e->getMessage());
       }
   }
   ```

5. **Update Drivers and Libraries**:
   Ensure that you are using the latest version of the fingerprint device drivers and any associated libraries. Sometimes, bugs in the device drivers or libraries can cause unexpected null reference errors.

6. **Consult Documentation**:
   Review the documentation provided by the fingerprint device manufacturer. There may be specific initialization steps or common issues documented that can help you resolve the problem.

If you provide more details about the specific code or the context in which this error is occurring, I can give more targeted advice.