GTA IV has a built in feature to stop anyone from doing this. Even so, you can still install external saves by deleting dsound.dll and replacing it with xlive.dll, included with LCPD:FR. Keep in mind, though, this disables online multiplayer.
However, saves while on duty shouldn't cause a problem anymore. Your crash probably has to do with something else. First, try the following common fixes:
Make sure you aren't using a crack
Update GTA IV to 1.0.7.0 (the latest version of the game, or 1.1.2.0 for EFLC) - This sometimes fixes a wide variety of issues
Make sure that you run the game as an administrator.
Make sure that your GTA IV root folder (and every subdirectory) is not set to "read-only"
Ensure that ScriptHook.dll, ScriptHookDotNet.asi and Dsound.dll OR Xlive.dll are in the GTA IV ROOT folder
Ensure that there is a folder titled 'LCPDFR' in the ROOT GTA IV folder.
Check that all the above files exist, if they do not, reinstall LCPDFR with the MANUAL installation.
Ensure that you are using up to date versions of every LCPDFR dependency and LCPDFR itself.
Install these: .Net Framework 4.5, Microsoft Visual C++ Redist 2012
If your problem has been solved after following these basic instructions, hit 'Mark Solved' next to this post.
If not, please now reply to this thread, and make sure that you do the following:
Give a clear, detailed description of the problem.
Explain any circumstances surrounding the problem.
State any solutions you have tried.
Post ALL relevant logs (LCPDFR.log, Scripthook.log, ScriptHookDotNet.log, AdvancedHook.log).
Include as much information as possible.
Post your system specifications.
Post what other mods you are using. This includes car and script mods.
If this post solved your problem, please hit the "Mark Solved" button next to this post.