'java.lang.String java.lang.Object.toString()' on a null object reference

ProductRef.child(productRandomKey).updateChildren(productMap)
                .addOnCompleteListener(new OnCompleteListener<Void>() {
                    @Override
                    public void onComplete(@NonNull Task<Void> task) {

                        if(task.isSuccessful()){

                            downloadImageUrl = task.getResult().toString();  //it is the problem

                            Intent intent = new Intent(AdminAddNewProductActivity.this, AdminCategoryActivity.class);
                            startActivity(intent);

                            loadingBar.dismiss();
                            Toast.makeText(AdminAddNewProductActivity.this, "Product is added successfully...", Toast.LENGTH_SHORT).show();

                        }

                        else{

                            loadingBar.dismiss();
                            String message = task.getException().toString();
                            Toast.makeText(AdminAddNewProductActivity.this, "Error: "+message, Toast.LENGTH_SHORT).show();

                        }

                    }
                });

final StorageReference filePath = ProductImageRef.child(ImageUri.getLastPathSegment() + productRandomKey + ".jpg");

    final UploadTask uploadTask = filePath.putFile(ImageUri);

downloadImageUrl = filePath.getDownloadUrl().toString();
return filePath.getDownloadUrl();

I have used these lines previously. it is occurring when I am trying to upload a picture the app crash.

You are calling toString(); on null object. So you can add some condition to check it. Change this:

downloadImageUrl = task.getResult().toString();

to

Result result = task.getResult();

if (result == null) {
    // TODO - handle null object here (e.g. show some placeholder)
} else {
    downloadImageUrl = result.toString();
}