"prerelease" key might not be in check if tracking stable releases

src/octoprint/plugins/softwareupdate/__init__.py

@@ -778,7 +778,7 @@ def _populated_check(self, target, check):
 						# if we are using the update_script, we need to set our update_branch and force
 						# to install the exact version we requested
 
-						if check["prerelease"]:
+						if check.get("prerelease", None):
 							# we are tracking prereleases => we want to be on the correct prerelease channel/branch
 							channel = check.get("prerelease_channel", None)
 							if channel: