Added fixes for crashing on bad filter

* Does not save profile in the SetFilter function.
* Saves profile on exit (exit from main loop).
* Clears filter string if it is found to be invalid in the Apply
  function.

cmd/mop/main.go

@@ -143,4 +143,5 @@ func main() {
 
 	profile := mop.NewProfile(*profileName)
 	mainLoop(screen, profile)
+        profile.Save()
 }

filter.go

@@ -50,13 +50,21 @@ func (filter *Filter) Apply(stocks []Stock) []Stock {
 		result, err := filter.profile.filterExpression.Evaluate(values)
 
 		if err != nil {
-			panic(err)
+                        // The filter isn't working, so reset to no filter.
+                        filter.profile.Filter = ""
+                        // Return an empty list.  The next main loop cycle will
+                        // show unfiltered.
+                        return filteredStocks
 		}
 
 		truthy, ok := result.(bool)
 
 		if !ok {
-			panic("Expression `" + filter.profile.Filter + "` should return a boolean value")
+                        // The filter isn't working, so reset to no filter.
+                        filter.profile.Filter = ""
+                        // Return an empty list.  The next main loop cycle will
+                        // show unfiltered.
+                        return filteredStocks
 		}
 
 		if truthy {

profile.go

@@ -142,5 +142,4 @@ func (profile *Profile) SetFilter(filter string) {
 	}
 
 	profile.Filter = filter
-	profile.Save()
 }