Error message display after delete a group of steps

I am encountering an issue when attempting to delete a group of steps in the Subject7 platform. After selecting "Container and steps" from the confirmation pop-up, the application displays an error message, and I am unable to proceed with the deletion. The error details are as follows:

Error Code: 500Message: Cannot invoke "com.subject7.proof.refactoring.dto.TreeNode.getParent()" because the return value of "com.subject7.proof.refactoring.dto.StepDtoHierarchy.lookupNode(com.subject7.proof.client.dto.executor.BaseStepDto)" is null

In addition, I occasionally receive a similar error message after running tests locally. Once the tests finish executing, the following error is displayed:

Error Code: 500Message: Cannot invoke "com.subject7.proof.refactoring.dto.TreeNode.getParent()" because the return value of "com.subject7.proof.refactoring.dto.StepDtoHierarchy.lookupNode(com.subject7.proof.client.dto.executor.BaseStepDto)" is null Cannot invoke "com.subject7.proof.refactoring.dto.TreeNode.getParent()" because the return value of "com.subject7.proof.refactoring.dto.StepDtoHierarchy.lookupNode(com.subject7.proof.client.dto.executor.BaseStepDto)" is null

Could you kindly assist in resolving this issue? Any insights or guidance would be greatly appreciated.