Fix two GUI bugs with uncaught exceptions

Closes #263, #219, #190 and potentially #229

src/control/RecentManager.cpp

@@ -180,7 +180,11 @@ GList* RecentManager::filterRecent(GList* items, bool xoj)
 		if (!ba::starts_with(uri, "file://")) continue;
 
 		using namespace boost::filesystem;
-		if (!exists(path(uri.substr(7)))) continue;	//substr is for removing uri's file://
+		try {
+			if (!exists(path(uri.substr(7)))) continue;	//substr is for removing uri's file://
+		} catch (boost::filesystem::filesystem_error) {
+			continue;
+		}
 
 		if (xoj && ba::ends_with(uri, ".xoj"))
 		{

src/gui/dialog/ExportDialog.cpp

@@ -386,7 +386,7 @@ bool ExportDialog::fileTypeByExtension()
 
 			gtk_tree_model_get(GTK_TREE_MODEL(this->typesModel), &iter, COL_EXTENSION, &currentExtension, -1);
 
-			if (extension == currentExtension)
+			if (currentExtension && extension == currentExtension)
 			{
 				gtk_tree_model_get(GTK_TREE_MODEL(this->typesModel), &iter, COL_TYPE, &this->type, -1);