Problem opening trees created from some alignments

I have made both Neighbor Joining and Maximum Likelihood trees using a set of 187 amino acid sequences using UGENE 1.15.0. Both trees were produced and I could navigate through them. They agreed well with previous trees I had made in other programs. I saved the whole project (alignment file and both tree files) and closed the program. When I opened the project later, I could access the alignment; however, I received the following log message when I tried to open either tree: 

"[ERROR][09:08] Task {Open new 'Phylogenetic tree viewer'} finished with error: Phylogenetic tree object not found"

I have tried to open the tree files directly and have tried the "Relocate..." function, but the error remains.