iPhoto to Photos: "Photos with unavailable original files cannot be opened?"

Hello:

I manually migrated my pictures from iPhoto to Photos and I was glad that it worked and I moved all of my pictures over. Therefore, I proceeded to delete my iPhoto library since iwas running very low on disk space and I had pretty much two copies of each photo- one in iPhoto and one in Photos. However, today, when I was looking at old pictures, I noticed that quite a number of them say "Missing file: Photos with unavailable original files cannot be opened. The original photo “IMG_xxxx.jpg” is either offline or cannot be found. Click “Find Original” to reconnect." I know it was probably not the best idea to delete the iPhoto library butI thought I got them all over to Photos. I never backed up iPhoto and I can still view the pictures on Photos but the annoying pop up is on the top of my screen still. How do I fix this?