repl: Fix crash on eval after %unload

Evaluating after %unload hit a bare assert, killing the whole REPL
with an AssertionError instead of just failing the command.

frida_tools/repl.py

@@ -503,6 +503,9 @@ class REPLApplication(ConsoleApplication):
                     fragments.append(("", "\n" + "\n".join(trimmed_stack)))
 
             output = FormattedText(fragments)
+        except NoScriptError:
+            self._print("No script loaded, use %reload to load one")
+            return success
         except frida.InvalidOperationError:
             return success
         if output != "undefined":
@@ -676,12 +679,14 @@ class REPLApplication(ConsoleApplication):
         return prompt_string
 
     def _evaluate_expression(self, expression: str) -> Tuple[str, bytes]:
-        assert self._script is not None
+        if self._script is None:
+            raise NoScriptError("no script loaded")
         result = self._script.exports_sync.frida_evaluate_expression(expression)
         return self._parse_evaluate_result(result)
 
     def _evaluate_quick_command(self, tokens: List[str]) -> Tuple[str, bytes]:
-        assert self._script is not None
+        if self._script is None:
+            raise NoScriptError("no script loaded")
         result = self._script.exports_sync.frida_evaluate_quick_command(tokens)
         return self._parse_evaluate_result(result)
 
@@ -1154,6 +1159,10 @@ class JavaScriptError(Exception):
         self.error = error
 
 
+class NoScriptError(frida.InvalidOperationError):
+    pass
+
+
 class DumbStdinReader:
     def __init__(self, valid_until: Callable[[], bool]) -> None:
         self._valid_until = valid_until