Git - nullSha1: contains entries pointing to null sha1

You could try, as suggested here, to remove the commit with the null sha1, using git filter-branch

git ls-tree f148931b374ad1aa9748b8f91bd48fcb44fb73b0 <an element> git filter-branch --index-filter 'git rm -r --cached --ignore-unmatch <an element path>' --prune-empty --tag-name-filter cat -- --all 

Replace <an element path> by the path relative to the root folder of the repo of the element returned by git ls-tree.

Just for the record, these days git filter-repo is the officially recommended alternative to git filter-branch, which is newer, safer and faster.

Tl;dr

First use git ls-tree --full-name <treeish> to find the path of the bad object(s)/file(s), then use git filter-repo --invert-paths to rewrite the history commits and remove the bad file(s).

Example usage of git filter-repo for removing objects

git filter-repo --invert-paths --path '<path-to-file1>' --path '<path-to-file2>` 

Or use option --path-glob, or --path-regex to match files with a pattern:

git filter-repo --invert-paths --path-glob '<glob>` --path-regex '<regex>' 

I got this error while trying to setup a project to use submodules. Getting rid of the submodules fixed the error.