Update buienradar sensors only after being added to HA (#131830)

* Update buienradar sensors only after being added to HA

* Move check to util

* Check for platform in sensor state property

* Move check to unit translation key property

* Add test for sensor check

* Properly handle added_to_hass

* Remove redundant comment

tests/components/sensor/test_init.py

@@ -545,6 +545,45 @@ async def test_translated_unit_with_native_unit_raises(
         assert entity0.entity_id is None
 
 
+async def test_unit_translation_key_without_platform_raises(
+    hass: HomeAssistant,
+) -> None:
+    """Test that unit translation key property raises if the entity has no platform yet."""
+
+    with patch(
+        "homeassistant.helpers.service.translation.async_get_translations",
+        return_value={
+            "component.test.entity.sensor.test_translation_key.unit_of_measurement": "Tests"
+        },
+    ):
+        entity0 = MockSensor(
+            name="Test",
+            native_value="123",
+            unique_id="very_unique",
+        )
+        entity0.entity_description = SensorEntityDescription(
+            "test",
+            translation_key="test_translation_key",
+        )
+        with pytest.raises(
+            ValueError,
+            match="cannot have a translation key for unit of measurement before "
+            "being added to the entity platform",
+        ):
+            unit = entity0.unit_of_measurement  # noqa: F841
+
+        setup_test_component_platform(hass, sensor.DOMAIN, [entity0])
+
+        assert await async_setup_component(
+            hass, "sensor", {"sensor": {"platform": "test"}}
+        )
+        await hass.async_block_till_done()
+
+        # Should not raise after being added to the platform
+        unit = entity0.unit_of_measurement  # noqa: F841
+        assert unit == "Tests"
+
+
 @pytest.mark.parametrize(
     (
         "device_class",