Chain keeps falling off chainring

I picked up an old Raleigh Sirocco from the 1980s ish and am attempting to convert it to single speed. I thought it was finished, but when taking it out for its first ride, the chain kept falling off the chainring in the front (yes, the chain is tightened properly). The bike has a 48/52T SR Apex crankset with two chainrings. I wanted to just leave it on and install the chain on the smaller 48T chainring to save having to replace these parts (especially since it has an obsolete 118 BCD). I installed a single speed 18T 3/32 freewheel in the rear.

Maybe I am unable to use only one chainring of the double set in this way? Originally there was a front derailleur, which I removed – is that why it keeps falling off? Perhaps I need to reinstall it to keep the chain "in place", even though I won’t be using it to shift gears anymore. Or alternatively, Should I remove both chainrings and install a new one instead (provided I can find one in the right size)? And if so – is there a difference between single speed and geared chainrings? Or do I need to replace the entire crankset? (I hope not…)

I checked the chainrings already and they do not seem bent / excessively worn.

Thanks a lot!