Impossible to close app without killing the session (Android)

I’m using Appium with python client. And I need to close app which is under test. Next step after closing app I need to open another application which is already pre-installed, click button and then app which was closed should open.

I’m doing it with command driver.close_app() but it’s killing a session and when I’m trying to open already installed app with command driver.start_activity() - I have exception

self = <json.decoder.JSONDecoder object at 0x10381a550>
s = “ERROR running Appium command: Cannot read property ‘startApp’ of null”
idx = 0

def raw_decode(self, s, idx=0):
    """Decode a JSON document from ``s`` (a ``str`` or ``unicode``
        beginning with a JSON document) and return a 2-tuple of the Python
        representation and the index in ``s`` where the document ended.

        This can be used to decode a JSON document from a string that may
        have extraneous data at the end.

        obj, end = self.scan_once(s, idx)
    except StopIteration:
      raise ValueError("No JSON object could be decoded")

E ValueError: No JSON object could be decoded

How to solve it? How to close app without killing session? Appium server is on remote machine and I cannot call adb command.