Trying to change a texture in a Material when the reference is missing causes an exception.

[neph1]

com.jme3.asset.AssetNotFoundException: Textures/the_missing_texture.png (Flipped) (Mipmapped)
	at com.jme3.asset.DesktopAssetManager.loadAsset(DesktopAssetManager.java:387)
	at com.jme3.asset.DesktopAssetManager.loadTexture(DesktopAssetManager.java:409)
	at com.jme3.asset.DesktopAssetManager.loadTexture(DesktopAssetManager.java:421)
	at com.jme3.gde.core.properties.TexturePropertyEditor.getValue(TexturePropertyEditor.java:89)

This can for example happen if you copy materials from one project to another.

Expected behavior: Ignore the missing texture and show the selector.

Work around: Use the 'source' window and edit it manually.