How to cope with "undefined is not an object" ?

I'm tiptoeing towards a solution, but stepped on a thumbtack:

The following snippet creates an error in the function GetTplParams

var template = "DDD-BibFM-tpl.fm";  // located in script-dir var outFile  = "DDD-BibFM.fm";      // located in the source dir   oDoc = OpenTemplate (); alert ("Is template open now?"); function OpenTemplate () {   var tplDir, tplFile, openParams, openReturnParams, oFile;   tplFile = "E:\\_DDDprojects\\FM+EN-escript\\escript\\DDD-BibFM-tpl.fm" // SimpleOpen does not take the tplFile, but opens the dir of the active document //  oFile = SimpleOpen (tplFile, true);   openParams = GetTplParams ();    openReturnParams =  new PropVals();    oFile = Open (tplFile, openParams, openReturnParams);    return oFile; }   function GetTplParams() {  // =>>> "undefined is not an object" on line 22 var params, i;  // Change the params   i = GetPropIndex(params, Constants.FS_RefFileNotFound);    params[i].propVal.ival = Constants.FV_AllowAllRefFilesUnFindable;    i = GetPropIndex(params, Constants.FS_FileIsOldVersion);    params[i].propVal.ival = Constants.FV_DoOK;    i = GetPropIndex(params, Constants.FS_FontNotFoundInDoc);    params[i].propVal.ival = Constants.FV_DoOK;    i = GetPropIndex(params, Constants.FS_LockCantBeReset);    params[i].propVal.ival = Constants.FV_DoOK;    i = GetPropIndex(params, Constants.FS_FileIsInUse);    params[i].propVal.ival = Constants.FV_OpenViewOnly;    i = GetPropIndex(params,Constants.FS_AlertUserAboutFailure);    params[i].propVal.ival=Constants.FV_DoCancel;  /*   i = GetPropIndex(params,Constants.FS_MakeVisible );    params[i].propVal.ival=false;  */   return (params);  }

(inserting JS code really has its quirks in this editor).