Absorb exception in isFoldableCodeAtRow and report assertion failure

Can’t figure out what’s going on in this bug, but we can hopefully make
it less damaging and collect more information via the new assertion
system.

Refs #5905

src/tokenized-buffer.coffee

@@ -292,7 +292,17 @@ class TokenizedBuffer extends Model
   # Returns a {Boolean} indicating whether the given buffer row starts
   # a a foldable row range due to the code's indentation patterns.
   isFoldableCodeAtRow: (row) ->
-    return false if @buffer.isRowBlank(row) or @tokenizedLineForRow(row).isComment()
+    # Investigating an exception that's occurring here due to the line being
+    # undefined. This should paper over the problem but we want to figure out
+    # what is happening:
+    tokenizedLine = @tokenizedLineForRow(row)
+    atom.assert tokenizedLine?, "TokenizedLine is defined", =>
+      metadata:
+        row: row
+        rowCount: @tokenizedLines.length
+    return false unless tokenizedLine?
+
+    return false if @buffer.isRowBlank(row) or tokenizedLine.isComment()
     nextRow = @buffer.nextNonBlankRow(row)
     return false unless nextRow?