Avoid closing preview panel when file switching

When we switch files with ANTLR4 preview panel opened, it's closed.
Now with this commit, we never close preview panel.
Each grammar have his preview conserved.

Author: OlivierCavadenti

src/main/java/org/antlr/intellij/plugin/ANTLRv4PluginController.java

@@ -278,11 +278,9 @@ public void currentEditorFileChangedEvent(VirtualFile oldFile, VirtualFile newFi
 		}
 		if ( newFile.getName().endsWith(".g") ) {
 			LOG.info("currentEditorFileChangedEvent ANTLR 4 cannot handle .g files, only .g4");
-			hidePreview();
 			return;
 		}
 		if ( !newFile.getName().endsWith(".g4") ) {
-			hidePreview();
 			return;
 		}