Remove debugger curframe attribute before using checkline()

Closes gh-10028

IPython/core/magics/execution.py

@@ -816,6 +816,11 b' python-profiler package from non-free.""")'
             self.shell.InteractiveTB.pdb = self.shell.InteractiveTB.debugger_cls()
             deb = self.shell.InteractiveTB.pdb
 
+        # deb.checkline() fails if deb.curframe exists but is None; it can
+        # handle it not existing. https://github.com/ipython/ipython/issues/10028
+        if hasattr(deb, 'curframe'):
+            del deb.curframe
+
         # reset Breakpoint state, which is moronically kept
         # in a class
         bdb.Breakpoint.next = 1