backgroundImage property Null safety

Field description: background image of the mixed video image. Recommended value: default value: null, indicating not to set the background image Note: you need to upload the background image in "Application Management" > "Function Configuration" > "Material Management" in the console in advance.
After the upload is successful, you can get the corresponding "image ID". Then, you need to convert it into a string and set it as backgroundImage.
For example, if the "image ID" is 63, you can set backgroundImage = "63";