Am assuming that you have written another EXT plugin to fix some NullPointerException existing in your previous plugin.
Its important to remember that once an EXT plugin has been deployed, some of its files are copied inside the Liferay installation, so the only way to remove its changes is to redeploy an unmodified Liferay application again.
Also, there is a tip at the end of this link
"After creating an Ext plugin, remove all of the files added by default that are not necessary for the extension. This is important because Liferay keeps track of the files deployed by each Ext plugin and it won't allow deploying two Ext plugins if they override the same file to avoid collisions. By removing any files not really necessary from an ext plugin it will be easier to use along with other Ext plugins."
These concepts might be helpful in finding your problem.