Don't write to stdin of closed process

Author: MatthijsBurgh

src/tue_get/installer_impl.py

@@ -155,7 +155,12 @@ def _set_target(self, target: str):
         self._current_target_dir = parent_target_dir
 
     def _out_handler(self, sub: BackgroundPopen, line: str) -> None:
-        def _write_stdin(msg) -> None:
+        def _write_stdin(msg: str) -> None:
+            if sub.returncode is not None:
+                self.tue_install_error(
+                    f"Cannot write to stdin of process {sub.pid} as it has already terminated {line=}"
+                )
+                return
             sub.stdin.write(f"{msg}\n")
             sub.stdin.flush()