clib.session: Refactor to simplify the checking of currently open ses…

…sion

pygmt/clib/session.py

@@ -160,7 +160,7 @@ def session_pointer(self) -> ctp.c_void_p:
             If trying to access without a currently open GMT session (i.e., outside of
             the context manager).
         """
-        if not hasattr(self, "_session_pointer") or self._session_pointer is None:
+        if getattr(self, "_session_pointer", None) is None:
             raise GMTCLibNoSessionError("No currently open GMT API session.")
         return self._session_pointer
 
@@ -338,19 +338,12 @@ def create(self, name: str):
         name
             A name for this session. Doesn't really affect the outcome.
         """
-        try:
-            # Won't raise an exception if there is a currently open session.
-            _ = self.session_pointer
-            # In this case, fail to create a new session until the old one is destroyed.
+        if getattr(self, "session_pointer", None) is not None:
             msg = (
                 "Failed to create a GMT API session: There is a currently open session."
                 " Must destroy it first."
             )
             raise GMTCLibError(msg)
-        # If the exception is raised, this means that there is no open session and we're
-        # free to create a new one.
-        except GMTCLibNoSessionError:
-            pass
 
         c_create_session = self.get_libgmt_func(
             "GMT_Create_Session",